diff --git a/apps/desktop/src/components/app/ChatApp.svelte b/apps/desktop/src/components/app/ChatApp.svelte
index b7cf318..9a4c6aa 100644
--- a/apps/desktop/src/components/app/ChatApp.svelte
+++ b/apps/desktop/src/components/app/ChatApp.svelte
@@ -53,9 +53,9 @@
       {:else}
         <div class="flex flex-1 items-center justify-center">
           <div class="text-center">
-            <p class="text-muted text-lg">チャンネルを選択</p>
+            <p class="text-muted text-lg">Select a Channel</p>
             <p class="text-muted mt-1 text-sm opacity-60">
-              左のサイドバーからチャンネルを選んでください
+              Choose a channel from the sidebar
             </p>
           </div>
         </div>
diff --git a/apps/desktop/src/components/app/OrganizationSidebar.svelte b/apps/desktop/src/components/app/OrganizationSidebar.svelte
index 72014fb..ab8b004 100644
--- a/apps/desktop/src/components/app/OrganizationSidebar.svelte
+++ b/apps/desktop/src/components/app/OrganizationSidebar.svelte
@@ -16,25 +16,25 @@
 <aside class="border-subtle bg-base-200 flex h-full w-72 flex-col border-r">
   <!-- Organization header -->
   <header
-    class="border-subtle flex items-center justify-between border-b px-4 py-3"
+    class="border-subtle flex items-center justify-between border-b px-4 py-4"
   >
     <div class="min-w-0 flex-1">
       <h1 class="truncate font-semibold tracking-tight">
         {organization?.name ?? "Loading..."}
       </h1>
     </div>
-    <div class="flex items-center gap-1">
+    <div class="flex items-center gap-2">
       <a
         href="/orgs/{organizationId}/settings"
         class="btn btn-ghost btn-sm btn-square hover-highlight"
-        title="組織設定"
+        title="Organization Settings"
       >
         <Settings class="text-muted size-4" />
       </a>
       <a
         href="/"
         class="btn btn-ghost btn-sm btn-square hover-highlight"
-        title="組織を切り替え"
+        title="Switch Organization"
       >
         <ArrowLeftRight class="text-muted size-4" />
       </a>
diff --git a/apps/desktop/src/components/channels/Channel.svelte b/apps/desktop/src/components/channels/Channel.svelte
index fcfdcf2..f6324ea 100644
--- a/apps/desktop/src/components/channels/Channel.svelte
+++ b/apps/desktop/src/components/channels/Channel.svelte
@@ -18,6 +18,10 @@
     () => selectedChannelId,
     () => organizationId,
   );
+
+  function autofocus(node: HTMLElement) {
+    node.focus();
+  }
 </script>
 
 <div class="flex h-full flex-col">
@@ -25,13 +29,13 @@
   <header
     class="border-subtle flex items-center justify-between border-b px-6 py-4"
   >
-    <div class="flex items-center gap-3">
-      <Hash class="size-5 opacity-50" />
+    <div class="flex items-center gap-4">
+      <Hash class="size-5 opacity-60" />
       <h1 class="text-base font-semibold">
         {controller.selectedChannel.data?.name ?? "..."}
       </h1>
       {#if controller.selectedChannel.data?.description}
-        <span class="hidden text-sm opacity-40 sm:inline">
+        <span class="hidden text-sm opacity-60 sm:inline">
           — {controller.selectedChannel.data.description}
         </span>
       {/if}
@@ -44,10 +48,11 @@
         >
           <input
             type="text"
-            placeholder="メッセージを検索..."
-            class="input input-sm input-bordered bg-base-200 w-56 pr-8 text-sm transition-all duration-150 focus:w-64"
+            placeholder="Search messages..."
+            class="input input-sm input-bordered bg-base-200 w-56 pr-8 text-sm transition-all duration-200 focus:w-64"
             bind:value={controller.searchQuery}
             onkeydown={(e) => e.key === "Enter" && controller.handleSearch()}
+            use:autofocus
           />
           {#if controller.isSearching}
             <span
@@ -57,12 +62,12 @@
         </div>
       {/if}
       <button
-        class="btn btn-ghost btn-sm btn-square transition-all duration-150"
-        title="検索"
+        class="btn btn-ghost btn-sm btn-square transition-all duration-200"
+        title="Search"
         onclick={() => (controller.showSearch = !controller.showSearch)}
       >
         <Search
-          class="size-5 opacity-50 transition-opacity duration-150 hover:opacity-80"
+          class="size-5 opacity-60 transition-opacity duration-200 hover:opacity-80"
         />
       </button>
     </div>
@@ -70,7 +75,7 @@
 
   <!-- Search results -->
   {#if controller.searchResults.length > 0}
-    <div class="border-subtle bg-base-200/50 border-b p-2">
+    <div class="border-subtle bg-base-200/50 border-b p-4">
       <SearchResults
         results={controller.searchResults}
         onResultClick={controller.handleResultClick}
diff --git a/apps/desktop/src/components/channels/ChannelList.svelte b/apps/desktop/src/components/channels/ChannelList.svelte
index a4cb7f0..39c017e 100644
--- a/apps/desktop/src/components/channels/ChannelList.svelte
+++ b/apps/desktop/src/components/channels/ChannelList.svelte
@@ -42,7 +42,7 @@
 <div class="flex h-full flex-col">
   <!-- Channels section -->
   <section class="flex-1 overflow-y-auto">
-    <header class="flex items-center justify-between px-3 py-2">
+    <header class="flex items-center justify-between px-4 py-2">
       <span class="text-muted text-xs font-medium tracking-wider uppercase">
         Channels
       </span>
@@ -59,7 +59,7 @@
           <a
             href={`/orgs/${organizationId}/chat/${channel.id}`}
             class={[
-              "group flex items-center gap-2 rounded px-2 py-1.5 text-sm transition-colors",
+              "group flex items-center gap-2 rounded px-2 py-2 text-sm transition-colors duration-200",
               active
                 ? "bg-primary/15 text-primary"
                 : "text-base-content/80 hover:bg-base-300 hover:text-base-content",
@@ -84,14 +84,14 @@
           </a>
         {/each}
       {:else}
-        <div class="text-muted px-2 py-4 text-center text-sm">
-          読み込み中...
+        <div class="text-muted px-2 py-4 text-center text-sm opacity-60">
+          Loading...
         </div>
       {/if}
 
       {#if channels.data?.length === 0}
-        <div class="text-muted px-2 py-4 text-center text-sm">
-          チャンネルがありません
+        <div class="text-muted px-2 py-4 text-center text-sm opacity-60">
+          No channels
         </div>
       {/if}
     </nav>
@@ -99,13 +99,13 @@
 
   <!-- DM section -->
   <section class="border-subtle border-t">
-    <header class="flex items-center justify-between px-3 py-2">
+    <header class="flex items-center justify-between px-4 py-2">
       <span class="text-muted text-xs font-medium tracking-wider uppercase">
         Direct Messages
       </span>
       <button
         class="btn btn-ghost btn-xs btn-square"
-        title="新しいDM"
+        title="New DM"
         onclick={() => (showUserSearch = !showUserSearch)}
       >
         <Plus class="text-muted size-4" />
@@ -132,10 +132,10 @@
   <footer class="border-subtle border-t p-2">
     <a
       href={`/orgs/${organizationId}/personalization`}
-      class="text-muted hover:bg-base-300 hover:text-base-content flex items-center gap-2 rounded px-2 py-1.5 text-sm transition-colors"
+      class="text-muted hover:bg-base-300 hover:text-base-content flex items-center gap-2 rounded px-2 py-2 text-sm transition-colors duration-200"
     >
       <User class="size-4" />
-      <span>個人設定</span>
+      <span>Settings</span>
     </a>
   </footer>
 </div>
diff --git a/apps/desktop/src/components/channels/CreateChannelButton.svelte b/apps/desktop/src/components/channels/CreateChannelButton.svelte
index f9865c7..56e96a1 100644
--- a/apps/desktop/src/components/channels/CreateChannelButton.svelte
+++ b/apps/desktop/src/components/channels/CreateChannelButton.svelte
@@ -42,7 +42,7 @@
 
 <button
   class="btn btn-ghost btn-xs btn-square"
-  title="新しいチャンネル"
+  title="New channel"
   onclick={() => {
     modalManager.dispatch(createChannelModalContent);
   }}
@@ -58,17 +58,17 @@
   >
     <input
       type="text"
-      placeholder="チャンネル名"
+      placeholder="Channel name"
       class="input input-bordered"
       bind:value={newChannelName}
     />
     {#if disabled}
       <button type="submit" class="btn btn-primary btn-sm" disabled>
-        作成中...
+        Creating...
         <span class="loading loading-spinner"></span>
       </button>
     {:else}
-      <button type="submit" class="btn btn-primary btn-sm">作成</button>
+      <button type="submit" class="btn btn-primary btn-sm">Create</button>
     {/if}
   </form>
 {/snippet}
diff --git a/apps/desktop/src/components/chat/EmojiButton.svelte b/apps/desktop/src/components/chat/EmojiButton.svelte
index 374297d..4a82d1b 100644
--- a/apps/desktop/src/components/chat/EmojiButton.svelte
+++ b/apps/desktop/src/components/chat/EmojiButton.svelte
@@ -10,4 +10,10 @@
   Button to toggle the emoji palette.
   Displays a smiling face emoji icon.
 -->
-<button class="btn btn-secondary self-end" {onclick}>😀</button>
+<button
+  class="btn btn-ghost btn-sm self-end"
+  {onclick}
+  aria-label="Open emoji picker"
+>
+  😀
+</button>
diff --git a/apps/desktop/src/components/chat/EmojiPalette.svelte b/apps/desktop/src/components/chat/EmojiPalette.svelte
index 5a289e0..82f821a 100644
--- a/apps/desktop/src/components/chat/EmojiPalette.svelte
+++ b/apps/desktop/src/components/chat/EmojiPalette.svelte
@@ -1,5 +1,6 @@
 <script lang="ts">
   import { Picker } from "emoji-picker-element";
+  import type { EmojiClickEvent } from "emoji-picker-element/shared";
 
   interface Props {
     onClose: () => void;
@@ -49,16 +50,18 @@
 
     const emojiPicker = document.querySelector("emoji-picker");
 
-    emojiPicker?.addEventListener("emoji-click", (event) => {
+    const handleEmojiClick = (event: EmojiClickEvent) => {
       const emoji = event.detail.unicode;
       if (!emoji) return;
       onEmojiSelected(emoji);
-    });
+    };
+
+    emojiPicker?.addEventListener("emoji-click", handleEmojiClick);
 
     return () => {
       document.removeEventListener("click", handleClickOutside);
       if (emojiPicker) {
-        emojiPicker.removeEventListener("emoji-click", () => {});
+        emojiPicker.removeEventListener("emoji-click", handleEmojiClick);
       }
     };
   });
diff --git a/apps/desktop/src/components/chat/FileAttachmentPreview.svelte b/apps/desktop/src/components/chat/FileAttachmentPreview.svelte
index 37175b9..3666d42 100644
--- a/apps/desktop/src/components/chat/FileAttachmentPreview.svelte
+++ b/apps/desktop/src/components/chat/FileAttachmentPreview.svelte
@@ -15,7 +15,7 @@
 -->
 {#if files.length > 0}
   <div class="space-y-2">
-    <h4 class="text-base-content/70 text-sm font-medium">添付ファイル:</h4>
+    <h4 class="text-base-content/70 text-sm font-medium">Attached Files:</h4>
     <div class="grid grid-cols-2 gap-2 sm:grid-cols-3 lg:grid-cols-4">
       {#each files as file, index (file.name)}
         <FilePreview
diff --git a/apps/desktop/src/components/chat/MessageDropdownContent.svelte b/apps/desktop/src/components/chat/MessageDropdownContent.svelte
index 2856ac3..66be7f5 100644
--- a/apps/desktop/src/components/chat/MessageDropdownContent.svelte
+++ b/apps/desktop/src/components/chat/MessageDropdownContent.svelte
@@ -20,34 +20,40 @@
     isPinned,
     isOwnMessage = false,
   }: Props = $props();
+
+  function handleDelete() {
+    if (window.confirm("Are you sure you want to delete this message?")) {
+      onDelete?.();
+    }
+  }
 </script>
 
 <ul
   class="menu dropdown-content bg-base-100 absolute z-[1] w-40 rounded-md border p-2 shadow"
 >
   <li>
-    <button onclick={onReply}>返信</button>
+    <button onclick={onReply}>Reply</button>
   </li>
   <li>
-    <button onclick={onAddReaction}>リアクションを付ける</button>
+    <button onclick={onAddReaction}>Add Reaction</button>
   </li>
   <li>
-    <button onclick={onShowReactions}>リアクションを表示</button>
+    <button onclick={onShowReactions}>Show Reactions</button>
   </li>
   <li>
     <button onclick={onPin}>
-      {isPinned ? "ピン解除" : "ピン留め"}
+      {isPinned ? "Unpin" : "Pin"}
     </button>
   </li>
   {#if isOwnMessage}
     {#if onEdit}
       <li>
-        <button onclick={onEdit}>編集</button>
+        <button onclick={onEdit}>Edit</button>
       </li>
     {/if}
     {#if onDelete}
       <li>
-        <button class="text-error" onclick={onDelete}>削除</button>
+        <button class="text-error" onclick={handleDelete}>Delete</button>
       </li>
     {/if}
   {/if}
diff --git a/apps/desktop/src/components/chat/MessageInput.svelte b/apps/desktop/src/components/chat/MessageInput.svelte
index 166ecec..b1bd1c7 100644
--- a/apps/desktop/src/components/chat/MessageInput.svelte
+++ b/apps/desktop/src/components/chat/MessageInput.svelte
@@ -32,11 +32,13 @@
   <!-- Reply banner -->
   {#if replyingTo}
     <div
-      class="border-subtle bg-base-200/50 flex items-center gap-2 border-b px-4 py-2 text-sm"
+      class="border-subtle bg-base-200/50 flex items-center gap-2 border-b px-4 py-3 text-sm"
     >
-      <span class="text-muted">返信先:</span>
+      <span class="text-muted opacity-60">Replying to:</span>
       <span class="text-primary font-medium">{replyingTo.author}</span>
-      <span class="text-muted flex-1 truncate">{replyingTo.content}</span>
+      <span class="text-muted flex-1 truncate opacity-60"
+        >{replyingTo.content}</span
+      >
       <button
         class="btn btn-ghost btn-xs btn-square"
         onclick={() => (replyingTo = null)}
@@ -75,7 +77,7 @@
   {/if}
 
   <!-- Input area -->
-  <div class="flex items-end gap-2 p-3">
+  <div class="flex items-end gap-2 p-4">
     <div class="flex-1">
       <MessageTextarea
         bind:value={controller.messageContent}
@@ -87,7 +89,7 @@
       />
     </div>
 
-    <div class="flex items-center gap-1">
+    <div class="flex items-center gap-2">
       <EmojiButton
         onclick={(e) => {
           e.stopPropagation();
diff --git a/apps/desktop/src/components/chat/MessageInputController.svelte.ts b/apps/desktop/src/components/chat/MessageInputController.svelte.ts
index b3a4def..c1ac169 100644
--- a/apps/desktop/src/components/chat/MessageInputController.svelte.ts
+++ b/apps/desktop/src/components/chat/MessageInputController.svelte.ts
@@ -87,7 +87,6 @@ export class MessageInputController {
     await this.api.sendMessageMutation.run({
       channelId: this.channelId,
       content: this.messageContent.trim() || "",
-      author: this.api.identity.data?.name || "unregistered",
       parentId: this.replyingTo?.id ?? undefined,
       attachments,
       voteId,
diff --git a/apps/desktop/src/components/chat/MessageItem.svelte b/apps/desktop/src/components/chat/MessageItem.svelte
index c2085a7..4f34eb3 100644
--- a/apps/desktop/src/components/chat/MessageItem.svelte
+++ b/apps/desktop/src/components/chat/MessageItem.svelte
@@ -52,7 +52,7 @@
   <!-- Reply reference -->
   {#if parentMessage}
     <div
-      class="mb-2 flex items-center gap-1.5 text-xs opacity-60 transition-opacity duration-150 group-hover:opacity-80"
+      class="mb-2 flex items-center gap-2 text-xs opacity-60 transition-opacity duration-200 group-hover:opacity-80"
     >
       <Reply class="size-3" />
       <span class="text-primary/80 font-medium">{parentMessage.author}</span>
@@ -61,22 +61,22 @@
   {/if}
 
   <!-- Message header -->
-  <div class="flex items-baseline gap-2.5">
+  <div class="flex items-baseline gap-2">
     <span class="text-primary text-sm font-semibold">{message.author}</span>
     <span
-      class="text-xs opacity-40 transition-opacity duration-150 group-hover:opacity-60"
+      class="text-xs opacity-60 transition-opacity duration-200 group-hover:opacity-80"
       >{formatTime(message.createdAt)}</span
     >
     {#if isEdited}
       <span
-        class="text-xs opacity-30 transition-opacity duration-150 group-hover:opacity-50"
-        >(編集済み)</span
+        class="text-xs opacity-60 transition-opacity duration-200 group-hover:opacity-80"
+        >(edited)</span
       >
     {/if}
     {#if message.pinnedAt}
-      <span class="text-warning flex items-center gap-1 text-xs opacity-70">
+      <span class="text-warning flex items-center gap-1 text-xs opacity-80">
         <Pin class="size-3" />
-        ピン留め
+        Pinned
       </span>
     {/if}
   </div>
@@ -85,7 +85,7 @@
   {#if isEditing}
     <div class="mt-3 space-y-2">
       <textarea
-        class="textarea textarea-bordered bg-base-300 w-full text-sm transition-all duration-150"
+        class="textarea textarea-bordered bg-base-300 w-full text-sm transition-all duration-200"
         value={editedContent}
         oninput={(e) =>
           onEditChange?.(
@@ -96,22 +96,22 @@
         rows="3"
       ></textarea>
       <div class="flex gap-2">
-        <button class="btn btn-primary btn-xs" onclick={onEditSave}>保存</button
+        <button class="btn btn-primary btn-xs" onclick={onEditSave}>Save</button
         >
         <button class="btn btn-ghost btn-xs" onclick={onEditCancel}>
-          キャンセル
+          Cancel
         </button>
       </div>
     </div>
   {:else}
-    <div class="mt-1.5 text-sm leading-relaxed whitespace-pre-wrap opacity-90">
+    <div class="mt-2 text-sm leading-relaxed whitespace-pre-wrap opacity-80">
       {message.content}
     </div>
   {/if}
 
   <!-- Attachments -->
   {#if message.attachments && message.attachments.length > 0}
-    <div class="mt-3 space-y-2">
+    <div class="mt-4 space-y-2">
       {#each message.attachments as fileId}
         <FileAttachment {fileId} compact={false} />
       {/each}
@@ -120,7 +120,7 @@
 
   <!-- Vote -->
   {#if message.vote}
-    <div class="mt-3">
+    <div class="mt-4">
       <VoteViewer voteId={message.vote} />
     </div>
   {/if}
@@ -128,12 +128,12 @@
   <!-- Reactions -->
   <ReactionButtons messageId={message.id} />
 
-  <!-- Actions (hover) -->
+  <!-- Actions (always visible for touch devices) -->
   <div
-    class="border-subtle bg-base-100 absolute top-2 right-4 flex gap-0.5 rounded border opacity-0 shadow-sm transition-all duration-200 group-hover:opacity-100"
+    class="border-subtle bg-base-100 absolute top-2 right-4 flex gap-1 rounded border opacity-0 shadow-sm transition-all duration-200 group-hover:opacity-100"
   >
     <button
-      class="btn btn-ghost btn-xs btn-square transition-colors duration-150"
+      class="btn btn-ghost btn-xs btn-square transition-colors duration-200"
       onclick={onDotsClick}
       title="メニュー"
     >
diff --git a/apps/desktop/src/components/chat/MessageList.svelte b/apps/desktop/src/components/chat/MessageList.svelte
index 42edf1a..591861d 100644
--- a/apps/desktop/src/components/chat/MessageList.svelte
+++ b/apps/desktop/src/components/chat/MessageList.svelte
@@ -101,12 +101,12 @@
       />
     {:else}
       <div class="flex h-full items-center justify-center py-16">
-        <p class="text-sm opacity-40">まだメッセージがありません</p>
+        <p class="text-sm opacity-60">No messages yet</p>
       </div>
     {/each}
   {:else}
     <div class="flex h-full items-center justify-center py-16">
-      <p class="text-sm opacity-40">メッセージを読み込み中...</p>
+      <p class="text-sm opacity-60">Loading messages...</p>
     </div>
   {/if}
 </div>
diff --git a/apps/desktop/src/components/chat/MessageTextarea.svelte b/apps/desktop/src/components/chat/MessageTextarea.svelte
index f44c4e6..cea174d 100644
--- a/apps/desktop/src/components/chat/MessageTextarea.svelte
+++ b/apps/desktop/src/components/chat/MessageTextarea.svelte
@@ -26,11 +26,12 @@
 -->
 <div class="flex-1 space-y-2">
   <textarea
-    placeholder="メッセージを入力... (Ctrl+Enterで送信、Enterで改行)"
+    placeholder="Type a message... (Ctrl+Enter to send, Enter for new line)"
     class="textarea textarea-bordered w-full resize-none"
     rows="2"
     bind:value
     {onkeydown}
+    aria-label="Message input"
   ></textarea>
 
   <!-- Action buttons -->
@@ -38,19 +39,19 @@
     <button
       class="btn btn-ghost btn-sm"
       onclick={ontoggleFileUploader}
-      title="ファイルを添付"
+      title="Attach file"
       type="button"
     >
       <Paperclip class="size-4" />
-      {showFileSelector ? "キャンセル" : "ファイル添付"}
+      {showFileSelector ? "Cancel" : "Attach File"}
     </button>
     <button
       class="btn btn-ghost btn-sm"
       onclick={ontoggleVoteMaker}
-      title="投票を作成"
+      title="Create poll"
       type="button"
     >
-      {showVoteMaker ? "キャンセル" : "投票を作成"}
+      {showVoteMaker ? "Cancel" : "Create Poll"}
     </button>
   </div>
 </div>
diff --git a/apps/desktop/src/components/chat/Personalization.svelte b/apps/desktop/src/components/chat/Personalization.svelte
index a3dfb25..9cb3014 100644
--- a/apps/desktop/src/components/chat/Personalization.svelte
+++ b/apps/desktop/src/components/chat/Personalization.svelte
@@ -4,48 +4,51 @@
   const personalization = usePersonalization();
 </script>
 
-<h2 class="py-2 text-center text-lg font-semibold">アイコンの変更</h2>
-<div class="flex justify-around">
-  <div class="w-32 flex-col">
-    <p class="mb-2 text-center">変更前</p>
-    <img src={personalization.imageURL} alt="googleアイコン" class="w-32" />
+<h2 class="py-2 text-center text-lg font-semibold">Change Profile Picture</h2>
+<div class="flex flex-col items-center gap-4 sm:flex-row sm:justify-around">
+  <div class="flex flex-col items-center">
+    <p class="mb-2 text-center text-sm">Current</p>
+    <img
+      src={personalization.imageURL}
+      alt="Current profile"
+      class="size-32 rounded-lg object-cover"
+    />
   </div>
   {#if personalization.changedImage}
-    <div class="w-32 flex-col">
-      <p class="mb-2 text-center">変更後</p>
-      <img src={personalization.changedImage} alt="変更後" class="w-32" />
+    <div class="flex flex-col items-center">
+      <p class="mb-2 text-center text-sm">New</p>
+      <img
+        src={personalization.changedImage}
+        alt="New profile"
+        class="size-32 rounded-lg object-cover"
+      />
     </div>
   {/if}
 </div>
 <input
   type="file"
-  class="file:bg-primary file:text-primary-content
-        text-sm
-        text-gray-500 file:mr-4 file:ml-2
-        file:rounded file:border-0
-        file:px-4
-        file:py-2 file:font-semibold"
+  class="file-input file-input-bordered file-input-primary mt-4 w-full"
   accept=".jpg, .png"
   onchange={personalization.handleFileChange}
 />
-<h2 class="py-2 text-center text-lg font-semibold">名前の変更</h2>
-<div class="flex justify-around">
-  <div>
-    <h3 class="text-center text-base">変更前</h3>
-    <h4 class="text-lg">{personalization.userName}</h4>
+<h2 class="py-2 text-center text-lg font-semibold">Change Name</h2>
+<div class="flex flex-col gap-4 sm:flex-row sm:justify-around">
+  <div class="flex flex-col items-center">
+    <h3 class="mb-2 text-center text-sm">Current</h3>
+    <p class="text-base font-medium">{personalization.userName}</p>
   </div>
-  <div>
-    <h3 class="text-center text-base">変更後</h3>
+  <div class="flex flex-col items-center">
+    <h3 class="mb-2 text-center text-sm">New</h3>
     <input
       type="text"
-      placeholder="ユーザー名"
-      class="input input-primary w-full"
+      placeholder="Username"
+      class="input input-bordered input-primary w-full sm:w-48"
       bind:value={personalization.changedUserName}
     />
   </div>
 </div>
 
 <button
-  class="btn btn-primary mt-auto mr-2 mb-2 ml-auto w-16"
-  onclick={personalization.save}>保存</button
+  class="btn btn-primary mt-4 ml-auto w-auto min-w-24"
+  onclick={personalization.save}>Save</button
 >
diff --git a/apps/desktop/src/components/chat/PinnedMessages.svelte b/apps/desktop/src/components/chat/PinnedMessages.svelte
index 61690c0..19c1550 100644
--- a/apps/desktop/src/components/chat/PinnedMessages.svelte
+++ b/apps/desktop/src/components/chat/PinnedMessages.svelte
@@ -61,26 +61,26 @@
 </script>
 
 {#if messages.length > 0}
-  <div class="bg-base-200 border-b p-2">
+  <div class="bg-base-200 border-b p-4">
     <div class="text-warning mb-2 flex items-center gap-2">
       <Pin class="size-4" />
-      <span class="text-sm font-semibold">ピン留めされたメッセージ</span>
+      <span class="text-sm font-semibold">Pinned Messages</span>
     </div>
-    <div class="space-y-1">
+    <div class="space-y-2">
       {#each messages as message}
         <button
-          class="hover:bg-base-300 w-full rounded p-2 text-left transition-colors"
+          class="hover:bg-base-300 w-full rounded p-2 text-left transition-colors duration-200"
           onclick={() => scrollToMessage(message.id)}
         >
           <div class="flex items-baseline gap-2">
             <span class="text-primary text-sm font-semibold">
               {message.author}
             </span>
-            <span class="text-base-content/60 text-xs">
+            <span class="text-base-content text-xs opacity-60">
               {formatTime(message.createdAt)}
             </span>
           </div>
-          <div class="text-base-content/80 truncate text-sm">
+          <div class="text-base-content truncate text-sm opacity-80">
             {message.content}
           </div>
         </button>
diff --git a/apps/desktop/src/components/chat/ReactionList.svelte b/apps/desktop/src/components/chat/ReactionList.svelte
index f63cf39..e4d97dd 100644
--- a/apps/desktop/src/components/chat/ReactionList.svelte
+++ b/apps/desktop/src/components/chat/ReactionList.svelte
@@ -87,7 +87,7 @@
                   reactionDetailsByEmoji.get(selectedEmoji)?.users ?? []}
                 {#if userIdsForSelectedEmoji.length === 0}
                   <tr>
-                    <td>No one has reacted with this emoji.</td>
+                    <td>No reactions with this emoji yet</td>
                   </tr>
                 {/if}
                 {#each userIdsForSelectedEmoji as userId}
@@ -111,7 +111,7 @@
     </div>
   {:else}
     <div class="flex h-96 items-center justify-center">
-      <p>There are no reactions yet.</p>
+      <p>No reactions yet</p>
     </div>
   {/if}
 </div>
diff --git a/apps/desktop/src/components/chat/ReplyBanner.svelte b/apps/desktop/src/components/chat/ReplyBanner.svelte
index 14455db..718634c 100644
--- a/apps/desktop/src/components/chat/ReplyBanner.svelte
+++ b/apps/desktop/src/components/chat/ReplyBanner.svelte
@@ -18,7 +18,7 @@
   class="bg-base-200 mb-2 flex items-center justify-between rounded-md p-2 text-sm"
 >
   <div class="text-base-content/70 truncate">
-    <span class="font-semibold">返信先:</span>
+    <span class="font-semibold">Replying to:</span>
     <span class="text-primary font-semibold">{replyingTo.author}</span>
     <span class="truncate">: {replyingTo.content}</span>
   </div>
diff --git a/apps/desktop/src/components/chat/SearchResults.svelte b/apps/desktop/src/components/chat/SearchResults.svelte
index bc6ad27..ed7ce8f 100644
--- a/apps/desktop/src/components/chat/SearchResults.svelte
+++ b/apps/desktop/src/components/chat/SearchResults.svelte
@@ -15,7 +15,7 @@
 >
   {#if results.length === 0}
     <div class="text-base-content/60 p-4 text-center text-sm">
-      検索結果がありません
+      No search results
     </div>
   {:else}
     {#each results as result}
diff --git a/apps/desktop/src/components/chat/SendButton.svelte b/apps/desktop/src/components/chat/SendButton.svelte
index 6e421c4..7111752 100644
--- a/apps/desktop/src/components/chat/SendButton.svelte
+++ b/apps/desktop/src/components/chat/SendButton.svelte
@@ -12,10 +12,15 @@
   Send button with loading state.
   Displays a spinner when processing the message send operation.
 -->
-<button class="btn btn-primary self-end" {onclick} {disabled}>
+<button
+  class="btn btn-primary btn-sm self-end"
+  {onclick}
+  {disabled}
+  aria-label="Send message"
+>
   {#if processing}
     <span class="loading loading-spinner loading-sm"></span>
   {:else}
-    送信
+    Send
   {/if}
 </button>
diff --git a/apps/desktop/src/components/chat/VoteMaker.svelte b/apps/desktop/src/components/chat/VoteMaker.svelte
index 2332b8e..12faf6e 100644
--- a/apps/desktop/src/components/chat/VoteMaker.svelte
+++ b/apps/desktop/src/components/chat/VoteMaker.svelte
@@ -16,54 +16,54 @@
   let newOption = $state("");
 </script>
 
-<div class="m-1 flex items-center">
-  <p class="text">投票のタイトル：</p>
+<div class="flex items-center gap-2">
+  <p class="text-sm opacity-80">Poll title:</p>
   <input
     type="text"
-    class="input input-sm input-bordered ml-2 w-128"
+    class="input input-sm input-bordered w-128"
     bind:value={vote.title}
   />
 </div>
 
-<div class="m-1 flex items-center">
-  <p class="text">一人が投票できる最大数：</p>
+<div class="flex items-center gap-2">
+  <p class="text-sm opacity-80">Max votes per person:</p>
   <input
     type="number"
-    class="input input-sm input-bordered ml-2 w-32"
+    class="input input-sm input-bordered w-32"
     bind:value={vote.maxVotes}
     onblur={() => {
       vote.maxVotes = vote.maxVotes ?? 0;
     }}
   />
 </div>
-<div class="max-h-32 overflow-auto">
+<div class="max-h-32 space-y-2 overflow-auto">
   {#each vote.voteOptions as option, i}
-    <div class="m-1 flex items-center">
-      <p class="text">{i}：{option}</p>
+    <div class="flex items-center gap-2">
+      <p class="text-sm">{i}: {option}</p>
       <button
-        class="btn btn-secondary ml-2"
+        class="btn btn-ghost btn-xs"
         onclick={() => {
           vote.voteOptions.splice(i, 1);
-        }}>削除</button
+        }}>Remove</button
       >
     </div>
   {/each}
 </div>
 
-<div class="m-1 flex items-center">
-  <p class="text">選択肢を追加：</p>
+<div class="flex items-center gap-2">
+  <p class="text-sm opacity-80">Add option:</p>
   <input
     type="text"
-    class="input input-sm input-bordered ml-2 w-128"
+    class="input input-sm input-bordered w-128"
     bind:value={newOption}
   />
   <button
-    class="btn btn-primary ml-2"
+    class="btn btn-primary btn-sm"
     onclick={() => {
       if (newOption.trim()) {
         vote.voteOptions.push(newOption);
         newOption = "";
       }
-    }}>追加</button
+    }}>Add</button
   >
 </div>
diff --git a/apps/desktop/src/components/chat/VoteViewer.svelte b/apps/desktop/src/components/chat/VoteViewer.svelte
index a3a828b..48047c4 100644
--- a/apps/desktop/src/components/chat/VoteViewer.svelte
+++ b/apps/desktop/src/components/chat/VoteViewer.svelte
@@ -6,40 +6,50 @@
   const controller = new VoteViewerController(() => ({ voteId }));
 </script>
 
-{#if controller.vote.data}
-  <div class="card bg-base-200 rounded p-2 shadow">
-    <h2 class="text-primary m-1 font-mono text-4xl">投票：</h2>
-    <h1 class="m-1 font-mono text-5xl">{controller.vote.data.title}</h1>
-    <p class="text-secondary m-1 font-mono">
-      一人の最大投票数：{controller.vote.data.maxVotes}票
+{#if controller.vote.isLoading}
+  <div class="text-sm opacity-60">Loading poll...</div>
+{:else if controller.vote.error}
+  <div class="text-error text-sm">Failed to load poll</div>
+{:else if controller.vote.data}
+  <div class="card bg-base-200 space-y-4 rounded p-4 shadow">
+    <h2 class="text-primary m-1 font-mono text-4xl">Poll:</h2>
+    <h1 class="font-mono text-5xl">{controller.vote.data.title}</h1>
+    <p class="text-secondary font-mono text-sm opacity-80">
+      Max votes per person: {controller.vote.data.maxVotes}
     </p>
-    {#each controller.vote.data.voteOptions as option, i}
-      {@const status = controller.clickableStatus(i)}
-      <div class="flex">
-        <p class="m-1 text-xl">
-          {#if controller.isResultVisible && controller.numbersOfVotersPerOption[i]}
-            {option}: {controller.numbersOfVotersPerOption[i]}人
-          {:else}
-            {option}
-          {/if}
-        </p>
-        <button
-          class={[
-            "btn m-1 ml-auto",
-            status === "can select" && "btn-primary",
-            status === "selected" && "btn-error",
-          ]}
-          disabled={status === "capped"}
-          onclick={() => {
-            controller.toggleSelectionOption(i);
-          }}
-        >
-          {controller.hasInSelectedOptions(i) ? "解除" : "選択"}
-        </button>
-      </div>
-    {/each}
-    <button class="btn btn-primary w-16" onclick={() => controller.castVote()}>
-      投票
+    <div class="space-y-2">
+      {#each controller.vote.data.voteOptions as option, i}
+        {@const status = controller.clickableStatus(i)}
+        <div class="flex items-center gap-2">
+          <p class="flex-1 text-xl">
+            {#if controller.isResultVisible && controller.numbersOfVotersPerOption[i]}
+              {option}: {controller.numbersOfVotersPerOption[i]} votes
+            {:else}
+              {option}
+            {/if}
+          </p>
+          <button
+            class={[
+              "btn btn-sm",
+              status === "can select" && "btn-primary",
+              status === "selected" && "btn-error",
+            ]}
+            disabled={status === "capped" || controller.isCasting}
+            onclick={() => {
+              controller.toggleSelectionOption(i);
+            }}
+          >
+            {controller.hasInSelectedOptions(i) ? "Deselect" : "Select"}
+          </button>
+        </div>
+      {/each}
+    </div>
+    <button
+      class="btn btn-primary btn-sm"
+      disabled={controller.isCasting}
+      onclick={() => controller.castVote()}
+    >
+      {controller.isCasting ? "Voting..." : "Vote"}
     </button>
   </div>
 {/if}
diff --git a/apps/desktop/src/components/chat/VoteViewer.svelte.ts b/apps/desktop/src/components/chat/VoteViewer.svelte.ts
index b238906..f2b09c6 100644
--- a/apps/desktop/src/components/chat/VoteViewer.svelte.ts
+++ b/apps/desktop/src/components/chat/VoteViewer.svelte.ts
@@ -17,6 +17,7 @@ export class VoteViewerController {
 
   vote: ReturnType<typeof useQuery<Vote>>;
   me: ReturnType<typeof useQuery<User>>;
+  isCasting = $state(false);
 
   constructor(props: () => { voteId: string }) {
     this.voteId = $derived(props().voteId);
@@ -108,9 +109,10 @@ export class VoteViewerController {
   }
 
   async castVote() {
-    if (this.me.data) {
+    if (this.me.data && !this.isCasting) {
       const voteRoute = getVote(this.api, this.voteId);
       if (voteRoute.cast) {
+        this.isCasting = true;
         try {
           const response = await voteRoute.cast.post({
             votedOptions: this.selectedOptions,
@@ -118,6 +120,8 @@ export class VoteViewerController {
           unwrapResponse(response);
         } catch (error) {
           console.error("Failed to cast vote:", error);
+        } finally {
+          this.isCasting = false;
         }
       }
     }
diff --git a/apps/desktop/src/components/chat/messageInputApi.svelte.ts b/apps/desktop/src/components/chat/messageInputApi.svelte.ts
index 5fab59a..45f5395 100644
--- a/apps/desktop/src/components/chat/messageInputApi.svelte.ts
+++ b/apps/desktop/src/components/chat/messageInputApi.svelte.ts
@@ -18,7 +18,6 @@ export class MessageInputApi {
     async (args: {
       channelId: string;
       content: string;
-      author: string;
       parentId?: string;
       attachments?: string[];
       voteId?: string;
diff --git a/apps/desktop/src/components/chat/messageList.ui.svelte.ts b/apps/desktop/src/components/chat/messageList.ui.svelte.ts
index a41ed8e..c14fe3c 100644
--- a/apps/desktop/src/components/chat/messageList.ui.svelte.ts
+++ b/apps/desktop/src/components/chat/messageList.ui.svelte.ts
@@ -16,8 +16,14 @@ export class MessageListUI {
     private mutations: MessageMutations,
     private refetch: () => Promise<void>,
   ) {
-    document.addEventListener("click", () => {
-      this.visibleDropdown = null;
+    $effect(() => {
+      const handleClick = () => {
+        this.visibleDropdown = null;
+      };
+      document.addEventListener("click", handleClick);
+      return () => {
+        document.removeEventListener("click", handleClick);
+      };
     });
   }
 
@@ -75,7 +81,7 @@ export class MessageListUI {
   }
 
   async handleDelete(messageId: string) {
-    if (!confirm("このメッセージを削除しますか？")) return;
+    if (!confirm("Are you sure you want to delete this message?")) return;
     await this.mutations.deleteMessage.run({ messageId });
     await this.refetch();
   }
diff --git a/apps/desktop/src/components/chat/messageList.websocket.svelte.ts b/apps/desktop/src/components/chat/messageList.websocket.svelte.ts
index bf2c3f5..f22ce3b 100644
--- a/apps/desktop/src/components/chat/messageList.websocket.svelte.ts
+++ b/apps/desktop/src/components/chat/messageList.websocket.svelte.ts
@@ -5,6 +5,18 @@ import {
   useWebSocket,
 } from "@/lib/websocket";
 
+function isMessage(obj: unknown): obj is Message {
+  return (
+    obj !== null &&
+    typeof obj === "object" &&
+    "id" in obj &&
+    "content" in obj &&
+    "userId" in obj &&
+    "channelId" in obj &&
+    "createdAt" in obj
+  );
+}
+
 /**
  * WebSocket event handlers for the message list.
  */
@@ -20,8 +32,8 @@ export function setupWebSocketHandlers(
   });
 
   useWebSocket("message:created", (event) => {
-    if (event.channelId === channelId) {
-      const newMessage = event.message as Message;
+    if (event.channelId === channelId && isMessage(event.message)) {
+      const newMessage = event.message;
       if (!messagesById.get(newMessage.id)) {
         setMessagesData([...messagesData, newMessage]);
       }
@@ -29,8 +41,8 @@ export function setupWebSocketHandlers(
   });
 
   useWebSocket("message:updated", (event) => {
-    if (event.channelId === channelId) {
-      const updated = event.message as Message;
+    if (event.channelId === channelId && isMessage(event.message)) {
+      const updated = event.message;
       setMessagesData(
         messagesData.map((m) => (m.id === updated.id ? updated : m)),
       );
diff --git a/apps/desktop/src/components/dms/DMList.svelte b/apps/desktop/src/components/dms/DMList.svelte
index eb0d832..ca9d744 100644
--- a/apps/desktop/src/components/dms/DMList.svelte
+++ b/apps/desktop/src/components/dms/DMList.svelte
@@ -17,18 +17,16 @@
 </script>
 
 <div class="flex flex-col">
-  <div class="border-base-300 border-b p-4">
-    <h3 class="text-base font-semibold">ダイレクトメッセージ</h3>
-  </div>
-
   <div class="flex-1 overflow-y-auto">
     {#if dms.data}
       {#each dms.data as dm (dm.id)}
         {@const active = selectedChannelId === dm.id}
         <a
           class={[
-            "border-base-300 block w-full border-b p-3 text-left",
-            active ? "bg-primary text-primary-content" : "hover:bg-base-300",
+            "block w-full rounded px-2 py-2 text-left text-sm transition-colors duration-200",
+            active
+              ? "bg-primary/15 text-primary"
+              : "text-base-content/80 hover:bg-base-300",
           ]}
           href={`/orgs/${organizationId}/chat/${dm.id}`}
         >
@@ -36,11 +34,13 @@
         </a>
       {/each}
     {:else}
-      <div class="text-base-content/60 p-4 text-center">読み込み中...</div>
+      <div class="text-base-content p-4 text-center opacity-60">
+        読み込み中...
+      </div>
     {/if}
 
     {#if dms.data && dms.data.length === 0}
-      <div class="text-base-content/60 p-4 text-center text-sm">
+      <div class="text-base-content p-4 text-center text-sm opacity-60">
         DMはまだありません
       </div>
     {/if}
diff --git a/apps/desktop/src/components/dms/UserSearch.svelte b/apps/desktop/src/components/dms/UserSearch.svelte
index 249a9b5..6856d4e 100644
--- a/apps/desktop/src/components/dms/UserSearch.svelte
+++ b/apps/desktop/src/components/dms/UserSearch.svelte
@@ -32,14 +32,23 @@
     }
   }
 
+  function isChannel(obj: unknown): obj is Channel {
+    return (
+      obj !== null &&
+      typeof obj === "object" &&
+      "id" in obj &&
+      "organizationId" in obj
+    );
+  }
+
   async function createDM(userId: string) {
     try {
       const response = await api.dms.post({
         otherUserId: userId,
         organizationId,
       });
-      const channel = unwrapResponse(response) as Channel | undefined;
-      if (channel?.id) {
+      const channel = unwrapResponse(response);
+      if (isChannel(channel) && channel.id) {
         window.location.href = `/orgs/${organizationId}/chat/${channel.id}`;
       }
     } catch (error) {
@@ -51,14 +60,16 @@
 <div class="p-4">
   <input
     type="text"
-    placeholder="メールアドレスで検索..."
+    placeholder="Search by email..."
     class="input input-bordered w-full"
     bind:value={searchQuery}
     oninput={searchUsers}
   />
 
   {#if isSearching}
-    <div class="text-base-content/60 mt-2 text-center text-sm">検索中...</div>
+    <div class="text-base-content/60 mt-2 text-center text-sm">
+      Searching...
+    </div>
   {/if}
 
   {#if users.length > 0}
diff --git a/apps/desktop/src/components/example/TaskList.svelte b/apps/desktop/src/components/example/TaskList.svelte
deleted file mode 100644
index 08ae534..0000000
--- a/apps/desktop/src/components/example/TaskList.svelte
+++ /dev/null
@@ -1,34 +0,0 @@
-<script lang="ts">
-  import { type Task } from "@packages/api-client";
-  import {
-    getApiClient,
-    getTask,
-    unwrapResponse,
-    useQuery,
-  } from "@/lib/api.svelte";
-  import TaskListSkin from "./TaskListSkin.svelte";
-
-  const api = getApiClient();
-
-  const todosQuery = useQuery<Task[]>(async () => {
-    const response = await api.tasks.get();
-    return unwrapResponse(response);
-  });
-
-  async function updateTodo(id: string, data: Partial<Task>) {
-    const response = await getTask(api, id).patch(data);
-    return unwrapResponse(response);
-  }
-
-  async function createTodo() {
-    unwrapResponse(await api.tasks.post({ text: "", assigner: "" }));
-  }
-</script>
-
-{#if todosQuery.data}
-  <TaskListSkin {updateTodo} {createTodo} todos={todosQuery.data} />
-{:else if todosQuery.isLoading}
-  <div class="flex items-center justify-center p-8">
-    <span class="loading loading-spinner loading-lg"></span>
-  </div>
-{/if}
diff --git a/apps/desktop/src/components/example/TaskListSkin.svelte b/apps/desktop/src/components/example/TaskListSkin.svelte
deleted file mode 100644
index f6c8cfb..0000000
--- a/apps/desktop/src/components/example/TaskListSkin.svelte
+++ /dev/null
@@ -1,60 +0,0 @@
-<script lang="ts">
-  import type { Task } from "@packages/api-client";
-  import BufferedTextInput from "$components/atoms/BufferedTextInput.svelte";
-
-  type Props = {
-    updateTodo: (id: string, data: Partial<Task>) => void;
-    createTodo: () => void;
-    todos: Task[];
-  };
-  const { updateTodo, createTodo, todos }: Props = $props();
-</script>
-
-<div class="bg-base-200 m-4 rounded-xl p-4">
-  <h1 class="p-5 text-center text-3xl">Task List</h1>
-  <ul class="list">
-    <li class="list-row">
-      <span class="list-col-grow text-center text-xl"> Task </span>
-      <span class="w-80 text-center text-xl"> Assigner </span>
-    </li>
-    {#each todos as todo}
-      <li class="list-row">
-        <input
-          type="checkbox"
-          class="checkbox checkbox-primary"
-          bind:checked={
-            () => todo.isCompleted,
-            (val) => {
-              updateTodo(todo.id, {
-                isCompleted: val,
-              });
-            }
-          }
-        />
-        <BufferedTextInput
-          class="input w-full"
-          bind:value={
-            () => todo.text,
-            (val) => {
-              updateTodo(todo.id, {
-                text: val,
-              });
-            }
-          }
-        />
-        <BufferedTextInput
-          class="input input-bordered"
-          bind:value={
-            () => todo.assigner,
-            (val) => {
-              updateTodo(todo.id, {
-                assigner: val,
-              });
-            }
-          }
-        />
-      </li>
-    {/each}
-  </ul>
-  <button class="btn btn-primary" onclick={createTodo}> New </button>
-</div>
diff --git a/apps/desktop/src/components/example/storybook/Button.svelte b/apps/desktop/src/components/example/storybook/Button.svelte
deleted file mode 100644
index 769f2ff..0000000
--- a/apps/desktop/src/components/example/storybook/Button.svelte
+++ /dev/null
@@ -1,40 +0,0 @@
-<script lang="ts">
-  import "./button.css";
-
-  interface Props {
-    /** Is this the principal call to action on the page? */
-    primary?: boolean;
-    /** What background color to use */
-    backgroundColor?: string;
-    /** How large should the button be? */
-    size?: "small" | "medium" | "large";
-    /** Button contents */
-    label: string;
-    /** The onclick event handler */
-    onclick?: () => void;
-  }
-
-  const {
-    primary = false,
-    backgroundColor,
-    size = "medium",
-    label,
-    ...props
-  }: Props = $props();
-
-  let mode = $derived(
-    primary ? "storybook-button--primary" : "storybook-button--secondary",
-  );
-  let style = $derived(
-    backgroundColor ? `background-color: ${backgroundColor}` : "",
-  );
-</script>
-
-<button
-  type="button"
-  class={["storybook-button", `storybook-button--${size}`, mode].join(" ")}
-  {style}
-  {...props}
->
-  {label}
-</button>
diff --git a/apps/desktop/src/components/example/storybook/Header.svelte b/apps/desktop/src/components/example/storybook/Header.svelte
deleted file mode 100644
index 6cdd250..0000000
--- a/apps/desktop/src/components/example/storybook/Header.svelte
+++ /dev/null
@@ -1,58 +0,0 @@
-<script lang="ts">
-  import Button from "./Button.svelte";
-  import "./header.css";
-
-  interface Props {
-    user?: { name: string };
-    onLogin?: () => void;
-    onLogout?: () => void;
-    onCreateAccount?: () => void;
-  }
-
-  const { user, onLogin, onLogout, onCreateAccount }: Props = $props();
-</script>
-
-<header>
-  <div class="storybook-header">
-    <div>
-      <svg
-        width="32"
-        height="32"
-        viewBox="0 0 32 32"
-        xmlns="http://www.w3.org/2000/svg"
-      >
-        <g fill="none" fill-rule="evenodd">
-          <path
-            d="M10 0h12a10 10 0 0110 10v12a10 10 0 01-10 10H10A10 10 0 010 22V10A10 10 0 0110 0z"
-            fill="#FFF"
-          />
-          <path
-            d="M5.3 10.6l10.4 6v11.1l-10.4-6v-11zm11.4-6.2l9.7 5.5-9.7 5.6V4.4z"
-            fill="#555AB9"
-          />
-          <path
-            d="M27.2 10.6v11.2l-10.5 6V16.5l10.5-6zM15.7 4.4v11L6 10l9.7-5.5z"
-            fill="#91BAF8"
-          />
-        </g>
-      </svg>
-      <h1>Acme</h1>
-    </div>
-    <div>
-      {#if user}
-        <span class="welcome">
-          Welcome, <b>{user.name}</b>!
-        </span>
-        <Button size="small" onclick={onLogout} label="Log out" />
-      {:else}
-        <Button size="small" onclick={onLogin} label="Log in" />
-        <Button
-          primary
-          size="small"
-          onclick={onCreateAccount}
-          label="Sign up"
-        />
-      {/if}
-    </div>
-  </div>
-</header>
diff --git a/apps/desktop/src/components/example/storybook/Page.svelte b/apps/desktop/src/components/example/storybook/Page.svelte
deleted file mode 100644
index d978362..0000000
--- a/apps/desktop/src/components/example/storybook/Page.svelte
+++ /dev/null
@@ -1,83 +0,0 @@
-<script lang="ts">
-  import Header from "./Header.svelte";
-  import "./page.css";
-
-  let user = $state<{ name: string }>();
-</script>
-
-<article>
-  <Header
-    {user}
-    onLogin={() => (user = { name: "Jane Doe" })}
-    onLogout={() => (user = undefined)}
-    onCreateAccount={() => (user = { name: "Jane Doe" })}
-  />
-
-  <section class="storybook-page">
-    <h2>Pages in Storybook</h2>
-    <p>
-      We recommend building UIs with a
-      <a
-        href="https://blog.hichroma.com/component-driven-development-ce1109d56c8e"
-        target="_blank"
-        rel="noopener noreferrer"
-      >
-        <strong>component-driven</strong>
-      </a>
-      process starting with atomic components and ending with pages.
-    </p>
-    <p>
-      Render pages with mock data. This makes it easy to build and review page
-      states without needing to navigate to them in your app. Here are some
-      handy patterns for managing page data in Storybook:
-    </p>
-    <ul>
-      <li>
-        Use a higher-level connected component. Storybook helps you compose such
-        data from the "args" of child component stories
-      </li>
-      <li>
-        Assemble data in the page component from your services. You can mock
-        these services out using Storybook.
-      </li>
-    </ul>
-    <p>
-      Get a guided tutorial on component-driven development at
-      <a
-        href="https://storybook.js.org/tutorials/"
-        target="_blank"
-        rel="noopener noreferrer"
-      >
-        Storybook tutorials
-      </a>
-      . Read more in the
-      <a
-        href="https://storybook.js.org/docs"
-        target="_blank"
-        rel="noopener noreferrer">docs</a
-      >
-      .
-    </p>
-    <div class="tip-wrapper">
-      <span class="tip">Tip</span>
-      Adjust the width of the canvas with the
-      <svg
-        width="10"
-        height="10"
-        viewBox="0 0 12 12"
-        xmlns="http://www.w3.org/2000/svg"
-      >
-        <g fill="none" fill-rule="evenodd">
-          <path
-            d="M1.5 5.2h4.8c.3 0 .5.2.5.4v5.1c-.1.2-.3.3-.4.3H1.4a.5.5 0
-            01-.5-.4V5.7c0-.3.2-.5.5-.5zm0-2.1h6.9c.3 0 .5.2.5.4v7a.5.5 0 01-1 0V4H1.5a.5.5 0
-            010-1zm0-2.1h9c.3 0 .5.2.5.4v9.1a.5.5 0 01-1 0V2H1.5a.5.5 0 010-1zm4.3 5.2H2V10h3.8V6.2z"
-            id="a"
-            fill="#999"
-          />
-        </g>
-      </svg>
-      Viewports addon in the toolbar
-    </div>
-  </section>
-</article>
diff --git a/apps/desktop/src/components/example/storybook/button.css b/apps/desktop/src/components/example/storybook/button.css
deleted file mode 100644
index 7efe955..0000000
--- a/apps/desktop/src/components/example/storybook/button.css
+++ /dev/null
@@ -1,30 +0,0 @@
-.storybook-button {
-  display: inline-block;
-  cursor: pointer;
-  border: 0;
-  border-radius: 3em;
-  font-weight: 700;
-  line-height: 1;
-  font-family: "Nunito Sans", "Helvetica Neue", Helvetica, Arial, sans-serif;
-}
-.storybook-button--primary {
-  background-color: #555ab9;
-  color: white;
-}
-.storybook-button--secondary {
-  box-shadow: rgba(0, 0, 0, 0.15) 0px 0px 0px 1px inset;
-  background-color: transparent;
-  color: #333;
-}
-.storybook-button--small {
-  padding: 10px 16px;
-  font-size: 12px;
-}
-.storybook-button--medium {
-  padding: 11px 20px;
-  font-size: 14px;
-}
-.storybook-button--large {
-  padding: 12px 24px;
-  font-size: 16px;
-}
diff --git a/apps/desktop/src/components/example/storybook/header.css b/apps/desktop/src/components/example/storybook/header.css
deleted file mode 100644
index ad77492..0000000
--- a/apps/desktop/src/components/example/storybook/header.css
+++ /dev/null
@@ -1,32 +0,0 @@
-.storybook-header {
-  display: flex;
-  justify-content: space-between;
-  align-items: center;
-  border-bottom: 1px solid rgba(0, 0, 0, 0.1);
-  padding: 15px 20px;
-  font-family: "Nunito Sans", "Helvetica Neue", Helvetica, Arial, sans-serif;
-}
-
-.storybook-header svg {
-  display: inline-block;
-  vertical-align: top;
-}
-
-.storybook-header h1 {
-  display: inline-block;
-  vertical-align: top;
-  margin: 6px 0 6px 10px;
-  font-weight: 700;
-  font-size: 20px;
-  line-height: 1;
-}
-
-.storybook-header button + button {
-  margin-left: 10px;
-}
-
-.storybook-header .welcome {
-  margin-right: 10px;
-  color: #333;
-  font-size: 14px;
-}
diff --git a/apps/desktop/src/components/example/storybook/page.css b/apps/desktop/src/components/example/storybook/page.css
deleted file mode 100644
index 2c9a9e0..0000000
--- a/apps/desktop/src/components/example/storybook/page.css
+++ /dev/null
@@ -1,68 +0,0 @@
-.storybook-page {
-  margin: 0 auto;
-  padding: 48px 20px;
-  max-width: 600px;
-  color: #333;
-  font-size: 14px;
-  line-height: 24px;
-  font-family: "Nunito Sans", "Helvetica Neue", Helvetica, Arial, sans-serif;
-}
-
-.storybook-page h2 {
-  display: inline-block;
-  vertical-align: top;
-  margin: 0 0 4px;
-  font-weight: 700;
-  font-size: 32px;
-  line-height: 1;
-}
-
-.storybook-page p {
-  margin: 1em 0;
-}
-
-.storybook-page a {
-  color: inherit;
-}
-
-.storybook-page ul {
-  margin: 1em 0;
-  padding-left: 30px;
-}
-
-.storybook-page li {
-  margin-bottom: 8px;
-}
-
-.storybook-page .tip {
-  display: inline-block;
-  vertical-align: top;
-  margin-right: 10px;
-  border-radius: 1em;
-  background: #e7fdd8;
-  padding: 4px 12px;
-  color: #357a14;
-  font-weight: 700;
-  font-size: 11px;
-  line-height: 12px;
-}
-
-.storybook-page .tip-wrapper {
-  margin-top: 40px;
-  margin-bottom: 40px;
-  font-size: 13px;
-  line-height: 20px;
-}
-
-.storybook-page .tip-wrapper svg {
-  display: inline-block;
-  vertical-align: top;
-  margin-top: 3px;
-  margin-right: 4px;
-  width: 12px;
-  height: 12px;
-}
-
-.storybook-page .tip-wrapper svg path {
-  fill: #1ea7fd;
-}
diff --git a/apps/desktop/src/components/organization/OrganizationSelector.svelte b/apps/desktop/src/components/organization/OrganizationSelector.svelte
index a3a9c71..fa283a0 100644
--- a/apps/desktop/src/components/organization/OrganizationSelector.svelte
+++ b/apps/desktop/src/components/organization/OrganizationSelector.svelte
@@ -18,10 +18,10 @@
 <div class="bg-base-100 flex min-h-screen items-center justify-center p-4">
   <div class="w-full max-w-md">
     <div class="mb-8 text-center">
-      <h1 class="text-base-content mb-2 text-4xl font-bold">組織を選択</h1>
-      <p class="text-base-content/70">
-        参加している組織からチャットする組織を選んでください
-      </p>
+      <h1 class="text-base-content mb-2 text-4xl font-bold">
+        Select Organization
+      </h1>
+      <p class="text-base-content/70">Choose an organization to chat with</p>
     </div>
 
     <div class="space-y-3">
@@ -57,8 +57,10 @@
 
     {#if organizations.data && organizations.data.length === 0}
       <div class="py-8 text-center">
-        <p class="text-base-content/60 mb-4">参加している組織がありません</p>
-        <a href="/orgs/new" class="btn btn-primary"> 新しい組織を作成 </a>
+        <p class="text-base-content/60 mb-4">No organizations available</p>
+        <a href="/orgs/new" class="btn btn-primary">
+          Create New Organization
+        </a>
       </div>
     {/if}
   </div>
diff --git a/apps/desktop/src/features/files/upload/FilePreview.svelte b/apps/desktop/src/features/files/upload/FilePreview.svelte
index 6b89d99..541ae9d 100644
--- a/apps/desktop/src/features/files/upload/FilePreview.svelte
+++ b/apps/desktop/src/features/files/upload/FilePreview.svelte
@@ -87,9 +87,9 @@
         controller.compact && "-top-1 -right-1",
       ]}
       onclick={controller.handleRemove}
-      title="削除"
+      title="Remove"
       type="button"
-      aria-label="ファイルを削除"
+      aria-label="Remove file"
     >
       <svg
         class="h-3 w-3"
diff --git a/apps/desktop/src/features/files/upload/SelectorDropzone.svelte b/apps/desktop/src/features/files/upload/SelectorDropzone.svelte
index 5eac810..3d5a40d 100644
--- a/apps/desktop/src/features/files/upload/SelectorDropzone.svelte
+++ b/apps/desktop/src/features/files/upload/SelectorDropzone.svelte
@@ -55,12 +55,12 @@
         </svg>
       </div>
       <p class="text-base-content/70 text-sm">
-        ファイルをドラッグ&ドロップ または <span
-          class="text-primary font-medium">クリックして選択</span
+        Drag and drop files or <span class="text-primary font-medium"
+          >click to select</span
         >
       </p>
       <p class="text-base-content/60 text-xs">
-        最大{MAX_FILES}ファイル、{formatFileSize(MAX_FILE_SIZE)}まで
+        Max {MAX_FILES} files, up to {formatFileSize(MAX_FILE_SIZE)}
       </p>
     </div>
   </div>
diff --git a/apps/desktop/src/features/files/upload/uploader.svelte.ts b/apps/desktop/src/features/files/upload/uploader.svelte.ts
index 7d19025..b7f9774 100644
--- a/apps/desktop/src/features/files/upload/uploader.svelte.ts
+++ b/apps/desktop/src/features/files/upload/uploader.svelte.ts
@@ -100,22 +100,18 @@ export function validate(...files: File[]) {
   const errors: Error[] = [];
 
   if (files.length > MAX_FILES) {
-    errors.push(new Error(`最大${MAX_FILES}ファイルまでアップロード可能です`));
+    errors.push(new Error(`Maximum ${MAX_FILES} files can be uploaded`));
     return { valid: [], errors };
   }
 
   for (const file of files) {
     if (file.size > MAX_FILE_SIZE) {
-      errors.push(
-        new Error(`${file.name}: ファイルサイズが大きすぎます（最大10MB）`),
-      );
+      errors.push(new Error(`${file.name}: File size too large (max 10MB)`));
       continue;
     }
 
     if (!ALLOWED_TYPES.includes(file.type)) {
-      errors.push(
-        new Error(`${file.name}: サポートされていないファイル形式です`),
-      );
+      errors.push(new Error(`${file.name}: Unsupported file format`));
       continue;
     }
 
diff --git a/apps/desktop/src/features/files/view/FileAttachment.svelte b/apps/desktop/src/features/files/view/FileAttachment.svelte
index 187dba6..fcb8def 100644
--- a/apps/desktop/src/features/files/view/FileAttachment.svelte
+++ b/apps/desktop/src/features/files/view/FileAttachment.svelte
@@ -39,7 +39,7 @@
         <button
           class="group relative block w-full cursor-pointer"
           onclick={controller.handleImageClick}
-          title="クリックして拡大表示"
+          title="Click to enlarge"
           type="button"
         >
           <img
@@ -107,9 +107,9 @@
           <button
             class="btn btn-ghost btn-sm"
             onclick={controller.handleDownload}
-            title="ダウンロード"
+            title="Download"
             type="button"
-            aria-label="ファイルをダウンロード"
+            aria-label="Download file"
           >
             <svg
               class="h-5 w-5"
@@ -133,7 +133,7 @@
   <div class="alert alert-error {controller.compact ? 'p-2' : 'p-3'}">
     <span>⚠️</span>
     <span class={controller.compact ? "text-xs" : "text-sm"}
-      >ファイルを読み込めませんでした</span
+      >Failed to load file</span
     >
   </div>
 {/if}
diff --git a/apps/desktop/src/lib/api.svelte.ts b/apps/desktop/src/lib/api.svelte.ts
index 82340cc..e5f2cbd 100644
--- a/apps/desktop/src/lib/api.svelte.ts
+++ b/apps/desktop/src/lib/api.svelte.ts
@@ -5,13 +5,12 @@ import {
   getFile,
   getMessage,
   getOrganization,
-  getTask,
   getVote,
 } from "@packages/api-client";
 import { goto } from "$app/navigation";
 
 // Re-export helper functions
-export { getChannel, getFile, getMessage, getOrganization, getTask, getVote };
+export { getChannel, getFile, getMessage, getOrganization, getVote };
 
 // Re-export hooks and utilities
 export { type QueryState, useMutation, useQuery } from "./api/hooks.svelte.ts";
diff --git a/apps/desktop/src/lib/api/utils.ts b/apps/desktop/src/lib/api/utils.ts
index cde396d..6e9bc46 100644
--- a/apps/desktop/src/lib/api/utils.ts
+++ b/apps/desktop/src/lib/api/utils.ts
@@ -1,12 +1,14 @@
+import type { ApiErrorResponse } from "@packages/api-client";
+
 /**
- * Helper to unwrap Eden Treaty responses.
- * Throws an error if the response contains an error or null data.
- * Also handles server-side error responses with { message: string } format.
+ * Helper function to extract data from Eden Treaty response
+ * Throws exception for error responses or null data
  */
 export function unwrapResponse<T>(response: {
   data?: unknown;
   error?: { status: unknown; value: unknown } | null;
 }): T {
+  // Eden Treaty error object
   if (response.error) {
     throw new Error(
       typeof response.error.value === "string"
@@ -14,17 +16,48 @@ export function unwrapResponse<T>(response: {
         : JSON.stringify(response.error.value),
     );
   }
+
+  // Data is undefined or null
   if (response.data === undefined || response.data === null) {
     throw new Error("Response data is undefined or null");
   }
-  // Handle server-side error responses
-  if (
-    typeof response.data === "object" &&
-    response.data !== null &&
-    "message" in response.data &&
-    Object.keys(response.data).length === 1
-  ) {
-    throw new Error((response.data as { message: string }).message);
+
+  // Detect server-side error response (supports new format)
+  if (isApiErrorResponse(response.data)) {
+    const error = response.data;
+    const errorMessage = error.code
+      ? `${error.message} (${error.code})`
+      : error.message;
+    throw new Error(errorMessage);
   }
+
   return response.data as T;
 }
+
+/**
+ * Determine if object is ApiErrorResponse
+ */
+function isApiErrorResponse(data: unknown): data is ApiErrorResponse {
+  if (typeof data !== "object" || data === null) return false;
+
+  const obj = data as Record<string, unknown>;
+
+  // message field is required
+  if (typeof obj.message !== "string") return false;
+
+  // code and details are optional
+  const hasCode = !("code" in obj) || typeof obj.code === "string";
+  const hasDetails = !("details" in obj) || obj.details !== undefined;
+
+  // Conditions for error response:
+  // 1. Has message field
+  // 2. Has code or details field, or only message
+  const keys = Object.keys(obj);
+  const isErrorResponse =
+    keys.includes("message") &&
+    hasCode &&
+    hasDetails &&
+    keys.every((key) => ["message", "code", "details"].includes(key));
+
+  return isErrorResponse;
+}
diff --git a/apps/desktop/src/routes/+page.svelte b/apps/desktop/src/routes/+page.svelte
index 18b4a07..c38961c 100644
--- a/apps/desktop/src/routes/+page.svelte
+++ b/apps/desktop/src/routes/+page.svelte
@@ -21,7 +21,7 @@
     <div class="hero-content text-center">
       <div class="max-w-md">
         <h1 class="text-7xl font-bold">Prism</h1>
-        <p class="py-6">The ultimate chat tool for engineers.</p>
+        <p class="py-6">The Ultimate Chat Tool for Engineers</p>
         <a href="/signin" class="btn btn-primary">Get Started</a>
       </div>
     </div>
diff --git a/apps/desktop/src/routes/orgs/[orgId]/settings/MemberList.svelte b/apps/desktop/src/routes/orgs/[orgId]/settings/MemberList.svelte
index 49616c6..51fd72a 100644
--- a/apps/desktop/src/routes/orgs/[orgId]/settings/MemberList.svelte
+++ b/apps/desktop/src/routes/orgs/[orgId]/settings/MemberList.svelte
@@ -15,10 +15,10 @@
 <div class="card bg-base-200 shadow-xl">
   <div class="card-body">
     <div class="mb-4 flex items-center justify-between">
-      <h2 class="card-title">メンバー</h2>
+      <h2 class="card-title">Members</h2>
       {#if organization?.permission === "admin"}
         <button class="btn btn-primary btn-sm" onclick={onAddMember}>
-          メンバーを追加
+          Add Member
         </button>
       {/if}
     </div>
@@ -53,7 +53,7 @@
                   class="btn btn-ghost btn-sm text-error"
                   onclick={() => onRemoveMember(member.userId)}
                 >
-                  削除
+                  Remove
                 </button>
               {/if}
             </div>
diff --git a/apps/desktop/src/routes/orgs/[orgId]/settings/OrganizationHeader.svelte b/apps/desktop/src/routes/orgs/[orgId]/settings/OrganizationHeader.svelte
index 0b80a83..79f9098 100644
--- a/apps/desktop/src/routes/orgs/[orgId]/settings/OrganizationHeader.svelte
+++ b/apps/desktop/src/routes/orgs/[orgId]/settings/OrganizationHeader.svelte
@@ -24,7 +24,7 @@
 
 <div class="mb-6">
   <a href={`/orgs/${organizationId}`} class="btn btn-ghost btn-sm mb-4">
-    ← 戻る
+    ← Back
   </a>
 
   {#if organization}
@@ -34,12 +34,12 @@
           <input
             bind:value={editForm.name}
             class="input input-bordered input-lg w-full max-w-md"
-            placeholder="組織名"
+            placeholder="Organization name"
           />
           <textarea
             bind:value={editForm.description}
             class="textarea textarea-bordered mt-2 w-full max-w-md"
-            placeholder="組織の説明"
+            placeholder="Organization description"
           ></textarea>
         {:else}
           <h1 class="text-base-content text-3xl font-bold">
@@ -56,10 +56,10 @@
       {#if organization.permission === "admin"}
         <div class="flex gap-2">
           {#if isEditing}
-            <button class="btn btn-primary" onclick={onUpdate}>保存</button>
-            <button class="btn btn-ghost" onclick={onCancel}>キャンセル</button>
+            <button class="btn btn-primary" onclick={onUpdate}>Save</button>
+            <button class="btn btn-ghost" onclick={onCancel}>Cancel</button>
           {:else}
-            <button class="btn btn-outline" onclick={onEdit}>編集</button>
+            <button class="btn btn-outline" onclick={onEdit}>Edit</button>
           {/if}
         </div>
       {/if}
diff --git a/apps/desktop/src/routes/orgs/[orgId]/settings/OrganizationInfo.svelte b/apps/desktop/src/routes/orgs/[orgId]/settings/OrganizationInfo.svelte
index 0aeb104..3ed8e5d 100644
--- a/apps/desktop/src/routes/orgs/[orgId]/settings/OrganizationInfo.svelte
+++ b/apps/desktop/src/routes/orgs/[orgId]/settings/OrganizationInfo.svelte
@@ -10,15 +10,15 @@
 
 <div class="card bg-base-200 shadow-xl">
   <div class="card-body">
-    <h2 class="card-title">組織情報</h2>
+    <h2 class="card-title">Organization Info</h2>
     {#if organization}
       <div class="space-y-3">
         <div>
-          <span class="font-semibold">作成日:</span>
-          {new Date(organization.createdAt).toLocaleDateString("ja-JP")}
+          <span class="font-semibold">Created:</span>
+          {new Date(organization.createdAt).toLocaleDateString("en-US")}
         </div>
         <div>
-          <span class="font-semibold">あなたの権限:</span>
+          <span class="font-semibold">Your Role:</span>
           <div class="badge badge-outline ml-2 capitalize">
             {organization.permission}
           </div>
diff --git a/apps/desktop/src/routes/orgs/[orgId]/settings/member-utils.ts b/apps/desktop/src/routes/orgs/[orgId]/settings/member-utils.ts
index 251cf74..acce00b 100644
--- a/apps/desktop/src/routes/orgs/[orgId]/settings/member-utils.ts
+++ b/apps/desktop/src/routes/orgs/[orgId]/settings/member-utils.ts
@@ -15,14 +15,14 @@ export async function searchAndAddMember(
   organizationId: string,
   currentMembers: OrganizationMember[] | undefined,
 ): Promise<void> {
-  const email = prompt("追加するメンバーのメールアドレスを入力してください");
+  const email = prompt("Enter the email address of the member to add");
   if (!email?.trim()) return;
 
   // Check if member already exists
   if (currentMembers) {
     for (const m of currentMembers) {
       if (m.user?.email === email) {
-        alert("そのメンバーはもう存在します");
+        alert("This member already exists");
         return;
       }
     }
@@ -38,25 +38,25 @@ export async function searchAndAddMember(
   );
 
   if (!users || !users.length) {
-    alert("ユーザーが見つかりませんでした");
+    alert("User not found");
     return;
   }
 
   if (users.length > 1) {
     alert(
-      "同じメールアドレスで登録されている人物が複数確認されました。開発者に報告してください。",
+      "Multiple users found with the same email address. Please report this to the developer.",
     );
     return;
   }
 
   const foundUser = users[0];
   if (!foundUser) {
-    alert("ユーザーが見つかりませんでした");
+    alert("User not found");
     return;
   }
 
   // Confirm and add user
-  const message = `以下のユーザーが見つかりました\n${foundUser.name}\n組織に追加しますか？`;
+  const message = `Found user:\n${foundUser.name}\nAdd to organization?`;
   const answer = confirm(message);
   if (answer) {
     const response = await getOrganization(api, organizationId).members.post({
diff --git a/apps/desktop/src/routes/orgs/[orgId]/settings/settings-controller.svelte.ts b/apps/desktop/src/routes/orgs/[orgId]/settings/settings-controller.svelte.ts
index 88dac15..9567439 100644
--- a/apps/desktop/src/routes/orgs/[orgId]/settings/settings-controller.svelte.ts
+++ b/apps/desktop/src/routes/orgs/[orgId]/settings/settings-controller.svelte.ts
@@ -56,7 +56,7 @@ export class SettingsController {
   }
 
   async handleRemoveMember(userId: string) {
-    if (confirm("このメンバーを削除しますか？")) {
+    if (confirm("Are you sure you want to remove this member?")) {
       try {
         await this.removeMember.run({
           organizationId: this.organizationId,
diff --git a/apps/desktop/src/routes/orgs/new/+page.svelte b/apps/desktop/src/routes/orgs/new/+page.svelte
index c67e77d..d6a29d9 100644
--- a/apps/desktop/src/routes/orgs/new/+page.svelte
+++ b/apps/desktop/src/routes/orgs/new/+page.svelte
@@ -27,7 +27,7 @@
       }
     } catch (error) {
       console.error("Failed to create organization:", error);
-      alert("組織の作成に失敗しました");
+      alert("Failed to create organization");
     }
   }
 
@@ -39,12 +39,14 @@
 <div class="container mx-auto max-w-md p-6">
   <div class="mb-6">
     <button class="btn btn-ghost btn-sm mb-4" onclick={goBack}>
-      ← ホームに戻る
+      ← Back to Home
     </button>
 
-    <h1 class="text-base-content mb-2 text-3xl font-bold">新しい組織を作成</h1>
+    <h1 class="text-base-content mb-2 text-3xl font-bold">
+      Create New Organization
+    </h1>
     <p class="text-base-content/70">
-      新しい組織を作成して、メンバーとコラボレーションを始めましょう
+      Create a new organization and start collaborating with members
     </p>
   </div>
 
@@ -53,15 +55,15 @@
       <form onsubmit={handleSubmit}>
         <div class="form-control mb-4">
           <label class="label" for="name">
-            <span class="label-text font-medium">組織名</span>
-            <span class="label-text-alt text-error">必須</span>
+            <span class="label-text font-medium">Organization Name</span>
+            <span class="label-text-alt text-error">Required</span>
           </label>
           <input
             id="name"
             bind:value={form.name}
             type="text"
             class="input input-bordered w-full"
-            placeholder="例: 株式会社サンプル"
+            placeholder="e.g., Acme Inc."
             required
             disabled={createOrganization.processing}
           />
@@ -69,14 +71,14 @@
 
         <div class="form-control mb-6">
           <label class="label" for="description">
-            <span class="label-text font-medium">組織の説明</span>
-            <span class="label-text-alt">任意</span>
+            <span class="label-text font-medium">Description</span>
+            <span class="label-text-alt">Optional</span>
           </label>
           <textarea
             id="description"
             bind:value={form.description}
             class="textarea textarea-bordered h-24 w-full"
-            placeholder="この組織について簡単に説明してください"
+            placeholder="Brief description of your organization"
             disabled={createOrganization.processing}
           ></textarea>
         </div>
@@ -88,7 +90,7 @@
             onclick={goBack}
             disabled={createOrganization.processing}
           >
-            キャンセル
+            Cancel
           </button>
           <button
             type="submit"
@@ -97,9 +99,9 @@
           >
             {#if createOrganization.processing}
               <span class="loading loading-spinner loading-sm"></span>
-              作成中...
+              Creating...
             {:else}
-              組織を作成
+              Create Organization
             {/if}
           </button>
         </div>
diff --git a/apps/desktop/src/routes/reset/+page.svelte b/apps/desktop/src/routes/reset/+page.svelte
deleted file mode 100644
index 5e5ea0b..0000000
--- a/apps/desktop/src/routes/reset/+page.svelte
+++ /dev/null
@@ -1,19 +0,0 @@
-<script lang="ts">
-  import { goto } from "$app/navigation";
-</script>
-
-<div class="hero bg-base-200 min-h-screen">
-  <div class="card bg-base-100 w-full max-w-sm shrink-0 shadow-2xl">
-    <div class="card-body">
-      <h1 class="text-2xl font-bold">Reset Password</h1>
-      <p class="text-base-content/70 text-sm">
-        Password reset is not yet implemented. Please contact support.
-      </p>
-      <div class="form-control mt-6">
-        <button class="btn btn-primary" onclick={() => goto("/signin")}>
-          Back to Sign In
-        </button>
-      </div>
-    </div>
-  </div>
-</div>
diff --git a/apps/desktop/src/routes/signin/SignInForm.svelte b/apps/desktop/src/routes/signin/SignInForm.svelte
index fed4e3d..89c8675 100644
--- a/apps/desktop/src/routes/signin/SignInForm.svelte
+++ b/apps/desktop/src/routes/signin/SignInForm.svelte
@@ -21,7 +21,7 @@
   }}
 >
   <h1 class="text-2xl font-bold">
-    {step === "signIn" ? "Sign In to Prism" : "Create an Account"}
+    {step === "signIn" ? "Sign In to Prism" : "Create Account"}
   </h1>
 
   <GoogleButton onclick={() => onSignIn("google")} />
@@ -72,7 +72,7 @@
     <input name="flow" type="hidden" value={step} />
     <button type="submit" class="btn btn-primary" disabled={isLoading}>
       {#if isLoading}<span class="loading loading-spinner"></span>{/if}
-      {step === "signIn" ? "Sign In" : "Continue"}
+      {step === "signIn" ? "Sign In" : "Next"}
     </button>
   </div>
 
@@ -81,13 +81,8 @@
   <div class="text-center">
     <button type="button" class="link-hover link" onclick={onToggleMode}>
       {step === "signIn"
-        ? "Don't have an account? Sign Up"
-        : "Already have an account? Sign In"}
+        ? "Don't have an account? Sign up"
+        : "Already have an account? Sign in"}
     </button>
   </div>
-  {#if step === "signIn"}
-    <div class="mt-4 text-center">
-      <a href="/reset" class="link-hover link text-sm">Forgot password?</a>
-    </div>
-  {/if}
 </form>
diff --git a/apps/desktop/src/routes/signin/VerificationForm.svelte b/apps/desktop/src/routes/signin/VerificationForm.svelte
index 7ffc3d4..fb0ce4c 100644
--- a/apps/desktop/src/routes/signin/VerificationForm.svelte
+++ b/apps/desktop/src/routes/signin/VerificationForm.svelte
@@ -17,9 +17,9 @@
     onSubmit(code);
   }}
 >
-  <h1 class="text-2xl font-bold">Enter Code</h1>
+  <h1 class="text-2xl font-bold">Enter Verification Code</h1>
   <p class="text-base-content/70 text-sm">
-    A code has been sent to <strong>{email}</strong>.
+    We sent a verification code to <strong>{email}</strong>
   </p>
   <div class="form-control">
     <label class="label" for="code">
@@ -38,7 +38,7 @@
   <div class="form-control mt-6">
     <button type="submit" class="btn btn-primary" disabled={isLoading}>
       {#if isLoading}<span class="loading loading-spinner"></span>{/if}
-      Confirm & Sign Up
+      Verify and Sign Up
     </button>
   </div>
 </form>
diff --git a/apps/desktop/src/routes/signout/+page.svelte b/apps/desktop/src/routes/signout/+page.svelte
index be06fb8..22e3495 100644
--- a/apps/desktop/src/routes/signout/+page.svelte
+++ b/apps/desktop/src/routes/signout/+page.svelte
@@ -34,7 +34,7 @@
 {:else}
   <div class="hero bg-base-200 min-h-screen">
     <div class="text-center">
-      <p>You are not signed in.</p>
+      <p>Not signed in</p>
       <a href="/signin" class="link mt-2">Go to Sign In</a>
     </div>
   </div>
diff --git a/apps/server/src/db/messages.ts b/apps/server/src/db/messages.ts
index 696cc7e..45562ac 100644
--- a/apps/server/src/db/messages.ts
+++ b/apps/server/src/db/messages.ts
@@ -41,19 +41,32 @@ export const messages = pgTable(
     updatedAt: timestamp("updated_at").defaultNow().notNull(),
     editedAt: timestamp("edited_at"),
   },
-  (table) => [index("messages_channel_idx").on(table.channelId)],
+  (table) => [
+    index("messages_channel_idx").on(table.channelId),
+    index("messages_user_idx").on(table.userId),
+    index("messages_parent_idx").on(table.parentId),
+    index("messages_created_at_idx").on(table.createdAt),
+    index("messages_pinned_at_idx").on(table.pinnedAt),
+  ],
 );
 
 // Message attachments (junction table)
-export const messageAttachments = pgTable("message_attachments", {
-  id: uuid("id").primaryKey().defaultRandom(),
-  messageId: uuid("message_id")
-    .notNull()
-    .references(() => messages.id, { onDelete: "cascade" }),
-  fileId: uuid("file_id")
-    .notNull()
-    .references(() => files.id, { onDelete: "cascade" }),
-});
+export const messageAttachments = pgTable(
+  "message_attachments",
+  {
+    id: uuid("id").primaryKey().defaultRandom(),
+    messageId: uuid("message_id")
+      .notNull()
+      .references(() => messages.id, { onDelete: "cascade" }),
+    fileId: uuid("file_id")
+      .notNull()
+      .references(() => files.id, { onDelete: "cascade" }),
+  },
+  (table) => [
+    index("message_attachments_message_idx").on(table.messageId),
+    index("message_attachments_file_idx").on(table.fileId),
+  ],
+);
 
 // Reactions
 export const reactions = pgTable(
diff --git a/apps/server/src/db/organizations.ts b/apps/server/src/db/organizations.ts
index 884ef8a..813ef5f 100644
--- a/apps/server/src/db/organizations.ts
+++ b/apps/server/src/db/organizations.ts
@@ -6,16 +6,20 @@ import { files } from "./files.ts";
 import { personalizations } from "./personalizations.ts";
 
 // Organizations
-export const organizations = pgTable("organizations", {
-  id: uuid("id").primaryKey().defaultRandom(),
-  name: text("name").notNull(),
-  description: text("description"),
-  ownerId: text("owner_id")
-    .notNull()
-    .references(() => users.id, { onDelete: "cascade" }),
-  createdAt: timestamp("created_at").defaultNow().notNull(),
-  updatedAt: timestamp("updated_at").defaultNow().notNull(),
-});
+export const organizations = pgTable(
+  "organizations",
+  {
+    id: uuid("id").primaryKey().defaultRandom(),
+    name: text("name").notNull(),
+    description: text("description"),
+    ownerId: text("owner_id")
+      .notNull()
+      .references(() => users.id, { onDelete: "cascade" }),
+    createdAt: timestamp("created_at").defaultNow().notNull(),
+    updatedAt: timestamp("updated_at").defaultNow().notNull(),
+  },
+  (table) => [index("organizations_owner_idx").on(table.ownerId)],
+);
 
 // Organization members
 export const organizationMembers = pgTable(
diff --git a/apps/server/src/db/personalizations.ts b/apps/server/src/db/personalizations.ts
index a10ceda..6fcfe6e 100644
--- a/apps/server/src/db/personalizations.ts
+++ b/apps/server/src/db/personalizations.ts
@@ -1,18 +1,29 @@
-import { pgTable, text, timestamp, uuid } from "drizzle-orm/pg-core";
+import { index, pgTable, text, timestamp, uuid } from "drizzle-orm/pg-core";
 import { users } from "./auth.ts";
 import { organizations } from "./organizations.ts";
 
 // Personalization
-export const personalizations = pgTable("personalizations", {
-  id: uuid("id").primaryKey().defaultRandom(),
-  userId: text("user_id")
-    .notNull()
-    .references(() => users.id, { onDelete: "cascade" }),
-  organizationId: uuid("organization_id")
-    .notNull()
-    .references(() => organizations.id, { onDelete: "cascade" }),
-  nickname: text("nickname").notNull(),
-  icon: text("icon"),
-  createdAt: timestamp("created_at").defaultNow().notNull(),
-  updatedAt: timestamp("updated_at").defaultNow().notNull(),
-});
+export const personalizations = pgTable(
+  "personalizations",
+  {
+    id: uuid("id").primaryKey().defaultRandom(),
+    userId: text("user_id")
+      .notNull()
+      .references(() => users.id, { onDelete: "cascade" }),
+    organizationId: uuid("organization_id")
+      .notNull()
+      .references(() => organizations.id, { onDelete: "cascade" }),
+    nickname: text("nickname").notNull(),
+    icon: text("icon"),
+    createdAt: timestamp("created_at").defaultNow().notNull(),
+    updatedAt: timestamp("updated_at").defaultNow().notNull(),
+  },
+  (table) => [
+    index("personalizations_user_idx").on(table.userId),
+    index("personalizations_org_idx").on(table.organizationId),
+    index("personalizations_user_org_idx").on(
+      table.userId,
+      table.organizationId,
+    ),
+  ],
+);
diff --git a/apps/server/src/domains/channels/routes.ts b/apps/server/src/domains/channels/routes.ts
index 67b4b40..706bf7b 100644
--- a/apps/server/src/domains/channels/routes.ts
+++ b/apps/server/src/domains/channels/routes.ts
@@ -2,80 +2,91 @@ import { desc, eq } from "drizzle-orm";
 import { Elysia, t } from "elysia";
 import { db } from "../../db/index.ts";
 import { channels } from "../../db/schema.ts";
+import {
+  BadRequestError,
+  ForbiddenError,
+  handleError,
+  NotFoundError,
+  UnauthorizedError,
+} from "../../lib/errors.ts";
 import { authMiddleware } from "../../middleware/auth.ts";
 import { getOrganizationPermissions } from "../organizations/permissions.ts";
 
 export const channelRoutes = new Elysia({ prefix: "/channels" })
   .use(authMiddleware)
   .get("/", async ({ user, query, set }) => {
-    if (!user) {
-      set.status = 401;
-      return { message: "Unauthorized" };
-    }
-    if (!query.organizationId) {
-      set.status = 400;
-      return { message: "organizationId is required" };
-    }
+    try {
+      if (!user) throw new UnauthorizedError();
+      if (!query.organizationId) {
+        throw new BadRequestError(
+          "organizationId is required",
+          "MISSING_ORGANIZATION_ID",
+        );
+      }
 
-    await getOrganizationPermissions(user.id, query.organizationId);
+      await getOrganizationPermissions(user.id, query.organizationId);
 
-    const channelList = await db
-      .select()
-      .from(channels)
-      .where(eq(channels.organizationId, query.organizationId))
-      .orderBy(desc(channels.createdAt));
+      const channelList = await db
+        .select()
+        .from(channels)
+        .where(eq(channels.organizationId, query.organizationId))
+        .orderBy(desc(channels.createdAt));
 
-    return channelList;
+      return channelList;
+    } catch (error) {
+      return handleError(error, set);
+    }
   })
   .get("/:id", async ({ user, params, set }) => {
-    if (!user) {
-      set.status = 401;
-      return { message: "Unauthorized" };
-    }
+    try {
+      if (!user) throw new UnauthorizedError();
 
-    const [channel] = await db
-      .select()
-      .from(channels)
-      .where(eq(channels.id, params.id))
-      .limit(1);
+      const [channel] = await db
+        .select()
+        .from(channels)
+        .where(eq(channels.id, params.id))
+        .limit(1);
 
-    if (!channel) {
-      set.status = 404;
-      return { message: "Channel not found" };
-    }
+      if (!channel) throw new NotFoundError("Channel", "CHANNEL_NOT_FOUND");
 
-    await getOrganizationPermissions(user.id, channel.organizationId);
+      await getOrganizationPermissions(user.id, channel.organizationId);
 
-    return channel;
+      return channel;
+    } catch (error) {
+      return handleError(error, set);
+    }
   })
   .post(
     "/",
     async ({ user, body, set }) => {
-      if (!user) {
-        set.status = 401;
-        return { message: "Unauthorized" };
-      }
+      try {
+        if (!user) throw new UnauthorizedError();
 
-      const perms = await getOrganizationPermissions(
-        user.id,
-        body.organizationId,
-      );
+        const perms = await getOrganizationPermissions(
+          user.id,
+          body.organizationId,
+        );
 
-      if (!perms.canCreateChannels) {
-        set.status = 403;
-        return { message: "Insufficient permissions" };
-      }
+        if (!perms.canCreateChannels) {
+          throw new ForbiddenError(
+            "Insufficient permissions",
+            "CANNOT_CREATE_CHANNEL",
+          );
+        }
 
-      const [channel] = await db
-        .insert(channels)
-        .values({
-          name: body.name,
-          description: body.description,
-          organizationId: body.organizationId,
-        })
-        .returning();
+        const [channel] = await db
+          .insert(channels)
+          .values({
+            name: body.name,
+            description: body.description,
+            organizationId: body.organizationId,
+          })
+          .returning();
 
-      return channel;
+        return channel;
+      } catch (error) {
+        return handleError(error, set);
+      }
     },
     {
       body: t.Object({
diff --git a/apps/server/src/domains/messages/create.routes.ts b/apps/server/src/domains/messages/create.routes.ts
index e38d2e7..8c2167e 100644
--- a/apps/server/src/domains/messages/create.routes.ts
+++ b/apps/server/src/domains/messages/create.routes.ts
@@ -2,6 +2,12 @@ import { eq } from "drizzle-orm";
 import { Elysia, t } from "elysia";
 import { db } from "../../db/index.ts";
 import { channels, messageAttachments, messages } from "../../db/schema.ts";
+import {
+  handleError,
+  InternalServerError,
+  NotFoundError,
+  UnauthorizedError,
+} from "../../lib/errors.ts";
 import { authMiddleware } from "../../middleware/auth.ts";
 import { wsManager } from "../../ws/manager.ts";
 import { requireOrganizationMembership } from "../organizations/permissions.ts";
@@ -12,59 +18,63 @@ import { requireOrganizationMembership } from "../organizations/permissions.ts";
 export const messageCreateRoutes = new Elysia().use(authMiddleware).post(
   "/",
   async ({ user, body, set }) => {
-    if (!user) {
-      set.status = 401;
-      return { message: "Unauthorized" };
-    }
+    try {
+      if (!user) throw new UnauthorizedError();
 
-    const [channel] = await db
-      .select()
-      .from(channels)
-      .where(eq(channels.id, body.channelId))
-      .limit(1);
+      const [channel] = await db
+        .select()
+        .from(channels)
+        .where(eq(channels.id, body.channelId))
+        .limit(1);
 
-    if (!channel) {
-      set.status = 404;
-      return { message: "Channel not found" };
-    }
+      if (!channel) throw new NotFoundError("Channel", "CHANNEL_NOT_FOUND");
 
-    await requireOrganizationMembership(user.id, channel.organizationId);
+      await requireOrganizationMembership(user.id, channel.organizationId);
 
-    const [message] = await db
-      .insert(messages)
-      .values({
-        channelId: body.channelId,
-        content: body.content,
-        author: body.author,
-        userId: user.id,
-        parentId: body.parentId,
-        voteId: body.voteId,
-      })
-      .returning();
+      const [message] = await db
+        .insert(messages)
+        .values({
+          channelId: body.channelId,
+          content: body.content,
+          author: user.name || user.email,
+          userId: user.id,
+          parentId: body.parentId,
+          voteId: body.voteId,
+        })
+        .returning();
 
-    // Handle attachments
-    if (body.attachments && body.attachments.length > 0 && message) {
-      await db.insert(messageAttachments).values(
-        body.attachments.map((fileId: string) => ({
-          messageId: message.id,
-          fileId,
-        })),
-      );
-    }
+      if (!message) {
+        throw new InternalServerError(
+          "Failed to create message",
+          "MESSAGE_CREATE_FAILED",
+        );
+      }
 
-    wsManager.broadcast(body.channelId, {
-      type: "message:created",
-      channelId: body.channelId,
-      message,
-    });
+      // Handle attachments
+      if (body.attachments && body.attachments.length > 0) {
+        await db.insert(messageAttachments).values(
+          body.attachments.map((fileId: string) => ({
+            messageId: message.id,
+            fileId,
+          })),
+        );
+      }
 
-    return message;
+      wsManager.broadcast(body.channelId, {
+        type: "message:created",
+        channelId: body.channelId,
+        message,
+      });
+
+      return message;
+    } catch (error) {
+      return handleError(error, set);
+    }
   },
   {
     body: t.Object({
       channelId: t.String(),
       content: t.String(),
-      author: t.String(),
       parentId: t.Optional(t.String()),
       attachments: t.Optional(t.Array(t.String())),
       voteId: t.Optional(t.String()),
diff --git a/apps/server/src/domains/messages/delete.routes.ts b/apps/server/src/domains/messages/delete.routes.ts
index 33f4b28..62bd861 100644
--- a/apps/server/src/domains/messages/delete.routes.ts
+++ b/apps/server/src/domains/messages/delete.routes.ts
@@ -2,6 +2,12 @@ import { eq } from "drizzle-orm";
 import { Elysia, t } from "elysia";
 import { db } from "../../db/index.ts";
 import { channels, messages } from "../../db/schema.ts";
+import {
+  ForbiddenError,
+  handleError,
+  NotFoundError,
+  UnauthorizedError,
+} from "../../lib/errors.ts";
 import { authMiddleware } from "../../middleware/auth.ts";
 import { wsManager } from "../../ws/manager.ts";
 import { requireOrganizationMembership } from "../organizations/permissions.ts";
@@ -12,49 +18,46 @@ import { requireOrganizationMembership } from "../organizations/permissions.ts";
 export const messageDeleteRoutes = new Elysia().use(authMiddleware).delete(
   "/:id",
   async ({ user, params, set }) => {
-    if (!user) {
-      set.status = 401;
-      return { message: "Unauthorized" };
-    }
+    try {
+      if (!user) throw new UnauthorizedError();
 
-    const [message] = await db
-      .select()
-      .from(messages)
-      .where(eq(messages.id, params.id))
-      .limit(1);
+      const [message] = await db
+        .select()
+        .from(messages)
+        .where(eq(messages.id, params.id))
+        .limit(1);
 
-    if (!message) {
-      set.status = 404;
-      return { message: "Message not found" };
-    }
+      if (!message) throw new NotFoundError("Message", "MESSAGE_NOT_FOUND");
 
-    if (message.userId !== user.id) {
-      set.status = 403;
-      return { message: "Forbidden: You can only delete your own messages" };
-    }
+      if (message.userId !== user.id) {
+        throw new ForbiddenError(
+          "You can only delete your own messages",
+          "CANNOT_DELETE_MESSAGE",
+        );
+      }
 
-    const [channel] = await db
-      .select()
-      .from(channels)
-      .where(eq(channels.id, message.channelId))
-      .limit(1);
+      const [channel] = await db
+        .select()
+        .from(channels)
+        .where(eq(channels.id, message.channelId))
+        .limit(1);
 
-    if (!channel) {
-      set.status = 404;
-      return { message: "Channel not found" };
-    }
+      if (!channel) throw new NotFoundError("Channel", "CHANNEL_NOT_FOUND");
 
-    await requireOrganizationMembership(user.id, channel.organizationId);
+      await requireOrganizationMembership(user.id, channel.organizationId);
 
-    await db.delete(messages).where(eq(messages.id, params.id));
+      await db.delete(messages).where(eq(messages.id, params.id));
 
-    wsManager.broadcast(message.channelId, {
-      type: "message:deleted",
-      channelId: message.channelId,
-      messageId: params.id,
-    });
+      wsManager.broadcast(message.channelId, {
+        type: "message:deleted",
+        channelId: message.channelId,
+        messageId: params.id,
+      });
 
-    return { success: true };
+      return { success: true };
+    } catch (error) {
+      return handleError(error, set);
+    }
   },
   {
     params: t.Object({
diff --git a/apps/server/src/domains/messages/reactions.ts b/apps/server/src/domains/messages/reactions.ts
index e3ec868..f776bdb 100644
--- a/apps/server/src/domains/messages/reactions.ts
+++ b/apps/server/src/domains/messages/reactions.ts
@@ -58,6 +58,11 @@ export const messageReactionRoutes = new Elysia()
         })
         .returning();
 
+      if (!reaction) {
+        set.status = 500;
+        return { message: "Failed to create reaction" };
+      }
+
       // Broadcast reaction:added event
       const [message] = await db
         .select({ channelId: messages.channelId })
diff --git a/apps/server/src/domains/messages/update.routes.ts b/apps/server/src/domains/messages/update.routes.ts
index a2bb7a9..60b684e 100644
--- a/apps/server/src/domains/messages/update.routes.ts
+++ b/apps/server/src/domains/messages/update.routes.ts
@@ -56,6 +56,11 @@ export const messageUpdateRoutes = new Elysia().use(authMiddleware).put(
       .where(eq(messages.id, params.id))
       .returning();
 
+    if (!updatedMessage) {
+      set.status = 500;
+      return { message: "Failed to update message" };
+    }
+
     wsManager.broadcast(message.channelId, {
       type: "message:updated",
       channelId: message.channelId,
diff --git a/apps/server/src/domains/organizations/permissions.ts b/apps/server/src/domains/organizations/permissions.ts
index 30987ab..80e2b81 100644
--- a/apps/server/src/domains/organizations/permissions.ts
+++ b/apps/server/src/domains/organizations/permissions.ts
@@ -1,6 +1,7 @@
 import { and, eq } from "drizzle-orm";
 import { db } from "../../db/index.ts";
 import { organizationMembers, organizations } from "../../db/schema.ts";
+import { ForbiddenError, NotFoundError } from "../../lib/errors.ts";
 
 export async function getOrganizationPermissions(
   userId: string,
@@ -13,7 +14,7 @@ export async function getOrganizationPermissions(
     .limit(1);
 
   if (!org) {
-    throw new Error("Organization not found");
+    throw new NotFoundError("Organization", "ORGANIZATION_NOT_FOUND");
   }
 
   const [membership] = await db
@@ -28,7 +29,10 @@ export async function getOrganizationPermissions(
     .limit(1);
 
   if (!membership) {
-    throw new Error("User is not a member of the organization");
+    throw new ForbiddenError(
+      "User is not a member of the organization",
+      "NOT_ORGANIZATION_MEMBER",
+    );
   }
 
   const isAdmin = membership.permission === "admin";
@@ -62,7 +66,10 @@ export async function requireOrganizationMembership(
     .limit(1);
 
   if (!membership) {
-    throw new Error("User is not a member of the organization");
+    throw new ForbiddenError(
+      "User is not a member of the organization",
+      "NOT_ORGANIZATION_MEMBER",
+    );
   }
 
   return membership;
diff --git a/apps/server/src/domains/tasks/routes.ts b/apps/server/src/domains/tasks/routes.ts
index 9ef7a09..d1317e4 100644
--- a/apps/server/src/domains/tasks/routes.ts
+++ b/apps/server/src/domains/tasks/routes.ts
@@ -12,7 +12,10 @@ export const taskRoutes = new Elysia({ prefix: "/tasks" })
       return { message: "Unauthorized" };
     }
 
-    const taskList = await db.select().from(tasks);
+    const taskList = await db
+      .select()
+      .from(tasks)
+      .where(eq(tasks.assigner, user.email));
 
     return taskList;
   })
@@ -50,6 +53,22 @@ export const taskRoutes = new Elysia({ prefix: "/tasks" })
         return { message: "Unauthorized" };
       }
 
+      // Check if task exists and belongs to current user
+      const [existingTask] = await db
+        .select()
+        .from(tasks)
+        .where(eq(tasks.id, params.id));
+
+      if (!existingTask) {
+        set.status = 404;
+        return { message: "Task not found" };
+      }
+
+      if (existingTask.assigner !== user.email) {
+        set.status = 403;
+        return { message: "Forbidden: You can only update your own tasks" };
+      }
+
       const updateData: {
         text?: string;
         isCompleted?: boolean;
diff --git a/apps/server/src/domains/users/routes.ts b/apps/server/src/domains/users/routes.ts
index fdd55d1..106a527 100644
--- a/apps/server/src/domains/users/routes.ts
+++ b/apps/server/src/domains/users/routes.ts
@@ -60,15 +60,20 @@ export const userRoutes = new Elysia({ prefix: "/users" })
   )
   .post(
     "/names",
-    async ({ body }) => {
+    async ({ user, body, set }) => {
+      if (!user) {
+        set.status = 401;
+        return { message: "Unauthorized" };
+      }
+
       const userList = await db
         .select()
         .from(users)
         .where(inArray(users.id, body.userIds));
 
       const userNames: Record<string, string> = {};
-      for (const user of userList) {
-        userNames[user.id] = user.name || "";
+      for (const dbUser of userList) {
+        userNames[dbUser.id] = dbUser.name || "";
       }
 
       return userNames;
@@ -81,7 +86,12 @@ export const userRoutes = new Elysia({ prefix: "/users" })
   )
   .post(
     "/nicknames",
-    async ({ body }) => {
+    async ({ user, body, set }) => {
+      if (!user) {
+        set.status = 401;
+        return { message: "Unauthorized" };
+      }
+
       const userList = await db
         .select()
         .from(users)
@@ -98,9 +108,9 @@ export const userRoutes = new Elysia({ prefix: "/users" })
         );
 
       const userNicknames: Record<string, string> = {};
-      for (const user of userList) {
-        const p = personalizationList.find((p) => p.userId === user.id);
-        userNicknames[user.id] = p?.nickname || user.name || "";
+      for (const dbUser of userList) {
+        const p = personalizationList.find((p) => p.userId === dbUser.id);
+        userNicknames[dbUser.id] = p?.nickname || dbUser.name || "";
       }
 
       return userNicknames;
@@ -112,7 +122,12 @@ export const userRoutes = new Elysia({ prefix: "/users" })
       }),
     },
   )
-  .get("/search", async ({ query }) => {
+  .get("/search", async ({ user, query, set }) => {
+    if (!user) {
+      set.status = 401;
+      return { message: "Unauthorized" };
+    }
+
     if (!query.email) return [];
 
     const userList = await db
diff --git a/apps/server/src/domains/votes/routes.ts b/apps/server/src/domains/votes/routes.ts
index 1040a80..99e0dac 100644
--- a/apps/server/src/domains/votes/routes.ts
+++ b/apps/server/src/domains/votes/routes.ts
@@ -47,9 +47,12 @@ export const voteRoutes = new Elysia({ prefix: "/votes" })
     },
     {
       body: t.Object({
-        title: t.String(),
-        maxVotes: t.Number(),
-        voteOptions: t.Array(t.String()),
+        title: t.String({ minLength: 1, maxLength: 200 }),
+        maxVotes: t.Number({ minimum: 1 }),
+        voteOptions: t.Array(t.String({ minLength: 1 }), {
+          minItems: 2,
+          maxItems: 20,
+        }),
       }),
     },
   )
@@ -72,6 +75,15 @@ export const voteRoutes = new Elysia({ prefix: "/votes" })
         return { message: "Vote not found" };
       }
 
+      // Validate votedOptions indices are within bounds
+      const invalidIndices = body.votedOptions.filter(
+        (index) => index < 0 || index >= vote.voteOptions.length,
+      );
+      if (invalidIndices.length > 0) {
+        set.status = 400;
+        return { message: "Invalid vote option indices" };
+      }
+
       // Check if user already voted
       const existingVoterIndex = vote.voters.findIndex(
         (v) => v.userId === user.id,
diff --git a/apps/server/src/lib/errors.ts b/apps/server/src/lib/errors.ts
new file mode 100644
index 0000000..94ae4f0
--- /dev/null
+++ b/apps/server/src/lib/errors.ts
@@ -0,0 +1,108 @@
+/**
+ * 標準的なAPIエラーレスポンスとエラークラス
+ */
+
+/**
+ * APIエラーレスポンスの標準フォーマット
+ */
+export interface ApiErrorResponse {
+  message: string;
+  code?: string;
+  details?: unknown;
+}
+
+/**
+ * HTTPエラーを表す基底クラス
+ */
+export class HttpError extends Error {
+  constructor(
+    public readonly status: number,
+    message: string,
+    public readonly code?: string,
+    public readonly details?: unknown,
+  ) {
+    super(message);
+    this.name = "HttpError";
+  }
+
+  toResponse(): ApiErrorResponse {
+    return {
+      message: this.message,
+      code: this.code,
+      details: this.details,
+    };
+  }
+}
+
+/**
+ * 400 Bad Request
+ */
+export class BadRequestError extends HttpError {
+  constructor(message: string, code?: string, details?: unknown) {
+    super(400, message, code, details);
+    this.name = "BadRequestError";
+  }
+}
+
+/**
+ * 401 Unauthorized
+ */
+export class UnauthorizedError extends HttpError {
+  constructor(message = "Unauthorized", code?: string) {
+    super(401, message, code);
+    this.name = "UnauthorizedError";
+  }
+}
+
+/**
+ * 403 Forbidden
+ */
+export class ForbiddenError extends HttpError {
+  constructor(message: string, code?: string) {
+    super(403, message, code);
+    this.name = "ForbiddenError";
+  }
+}
+
+/**
+ * 404 Not Found
+ */
+export class NotFoundError extends HttpError {
+  constructor(resource: string, code?: string) {
+    super(404, `${resource} not found`, code);
+    this.name = "NotFoundError";
+  }
+}
+
+/**
+ * 500 Internal Server Error
+ */
+export class InternalServerError extends HttpError {
+  constructor(message = "Internal server error", code?: string) {
+    super(500, message, code);
+    this.name = "InternalServerError";
+  }
+}
+
+/**
+ * HttpErrorをハンドリングしてElysiaレスポンスに変換
+ */
+export function handleError(
+  error: unknown,
+  set: { status?: number | string },
+): ApiErrorResponse {
+  if (error instanceof HttpError) {
+    set.status = error.status;
+    return error.toResponse();
+  }
+
+  // 通常のErrorの場合
+  if (error instanceof Error) {
+    set.status = 500;
+    return { message: error.message };
+  }
+
+  // 不明なエラー
+  set.status = 500;
+  return { message: "An unknown error occurred" };
+}
diff --git a/apps/server/src/ws/index.ts b/apps/server/src/ws/index.ts
index 2ed25d7..76f2f55 100644
--- a/apps/server/src/ws/index.ts
+++ b/apps/server/src/ws/index.ts
@@ -1,4 +1,8 @@
+import { eq } from "drizzle-orm";
 import { Elysia, t } from "elysia";
+import { db } from "../db/index.ts";
+import { channels } from "../db/schema.ts";
+import { requireOrganizationMembership } from "../domains/organizations/permissions.ts";
 import { authMiddleware } from "../middleware/auth.ts";
 import { wsManager } from "./manager.ts";
 import type { WsConnection } from "./types.ts";
@@ -13,20 +17,44 @@ const wsClientMessage = t.Union([
   t.Object({ type: t.Literal("ping") }),
 ]);
 
+const wsMessage = t.Object({
+  id: t.String(),
+  channelId: t.String(),
+  content: t.String(),
+  author: t.String(),
+  userId: t.String(),
+  parentId: t.Nullable(t.String()),
+  voteId: t.Nullable(t.String()),
+  pinnedAt: t.Nullable(t.Date()),
+  pinnedBy: t.Nullable(t.String()),
+  createdAt: t.Date(),
+  updatedAt: t.Date(),
+  editedAt: t.Nullable(t.Date()),
+});
+
+const wsReaction = t.Object({
+  id: t.String(),
+  messageId: t.String(),
+  userId: t.String(),
+  emoji: t.String(),
+  createdAt: t.Date(),
+});
+
 const wsServerMessage = t.Union([
   t.Object({ type: t.Literal("subscribed"), channelId: t.String() }),
   t.Object({ type: t.Literal("unsubscribed"), channelId: t.String() }),
   t.Object({ type: t.Literal("pong") }),
+  t.Object({ type: t.Literal("error"), message: t.String() }),
   t.Object({
     type: t.Literal("message:created"),
     channelId: t.String(),
-    message: t.Unknown(),
+    message: wsMessage,
   }),
   t.Object({
     type: t.Literal("message:updated"),
     channelId: t.String(),
     messageId: t.String(),
-    message: t.Unknown(),
+    message: wsMessage,
   }),
   t.Object({
     type: t.Literal("message:deleted"),
@@ -37,7 +65,7 @@ const wsServerMessage = t.Union([
     type: t.Literal("reaction:added"),
     channelId: t.String(),
     messageId: t.String(),
-    reaction: t.Unknown(),
+    reaction: wsReaction,
   }),
   t.Object({
     type: t.Literal("reaction:removed"),
@@ -69,17 +97,40 @@ export const wsRoutes = new Elysia().use(authMiddleware).ws("/ws", {
     ws.subscribe("global");
   },
 
-  message(ws, msg) {
+  async message(ws, msg) {
     const user = ws.data.user;
     if (!user) return;
 
     if (msg.type === "subscribe") {
       const connections = Array.from(wsManager.connections.values());
       const conn = connections.find((c) => c.user.id === user.id);
-      if (conn) {
-        wsManager.subscribe(conn.id, msg.channelId);
-        ws.send({ type: "subscribed", channelId: msg.channelId });
+      if (!conn) return;
+
+      // Lookup channel to verify organization membership
+      const [channel] = await db
+        .select()
+        .from(channels)
+        .where(eq(channels.id, msg.channelId))
+        .limit(1);
+
+      if (!channel) {
+        ws.send({ type: "error", message: "Channel not found" });
+        return;
       }
+
+      // Verify user is a member of the channel's organization
+      try {
+        await requireOrganizationMembership(user.id, channel.organizationId);
+      } catch {
+        ws.send({
+          type: "error",
+          message: "Not authorized to access this channel",
+        });
+        return;
+      }
+
+      wsManager.subscribe(conn.id, msg.channelId);
+      ws.send({ type: "subscribed", channelId: msg.channelId });
     } else if (msg.type === "unsubscribe") {
       const connections = Array.from(wsManager.connections.values());
       const conn = connections.find((c) => c.user.id === user.id);
diff --git a/apps/server/src/ws/types.ts b/apps/server/src/ws/types.ts
index 9ec7c92..ffdf394 100644
--- a/apps/server/src/ws/types.ts
+++ b/apps/server/src/ws/types.ts
@@ -1,30 +1,60 @@
 import type { AuthUser } from "../middleware/auth.ts";
 
+/**
+ * Message object sent in WebSocket events.
+ */
+export interface WsMessage {
+  id: string;
+  channelId: string;
+  content: string;
+  author: string;
+  userId: string;
+  parentId: string | null;
+  voteId: string | null;
+  pinnedAt: Date | null;
+  pinnedBy: string | null;
+  createdAt: Date;
+  updatedAt: Date;
+  editedAt: Date | null;
+}
+
+/**
+ * Reaction object sent in WebSocket events.
+ */
+export interface WsReaction {
+  id: string;
+  messageId: string;
+  userId: string;
+  emoji: string;
+  createdAt: Date;
+}
+
 /**
  * Control messages sent to specific clients.
  */
 export type WsControlMessage =
   | { type: "subscribed"; channelId: string }
   | { type: "unsubscribed"; channelId: string }
-  | { type: "pong" };
+  | { type: "pong" }
+  | { type: "error"; message: string };
 
 /**
  * Broadcast events sent to channel subscribers.
  */
 export type WsBroadcastEvent =
-  | { type: "message:created"; channelId: string; message: unknown }
+  | { type: "message:created"; channelId: string; message: WsMessage }
   | {
       type: "message:updated";
       channelId: string;
       messageId: string;
-      message: unknown;
+      message: WsMessage;
     }
   | { type: "message:deleted"; channelId: string; messageId: string }
   | {
       type: "reaction:added";
       channelId: string;
       messageId: string;
-      reaction: unknown;
+      reaction: WsReaction;
     }
   | {
       type: "reaction:removed";
diff --git a/packages/api-client/src/route-helpers.ts b/packages/api-client/src/route-helpers.ts
index 9efbf34..0f38d04 100644
--- a/packages/api-client/src/route-helpers.ts
+++ b/packages/api-client/src/route-helpers.ts
@@ -4,7 +4,6 @@ import type {
   FileRoute,
   MessagesRoute,
   OrganizationRoute,
-  TaskRoute,
   VoteRoute,
 } from "./route-types.ts";
 
@@ -13,35 +12,33 @@ import type {
  * Eden Treaty uses Proxy to enable dynamic route access at runtime.
  */
 
+type DynamicIndex<T> = Record<string, T>;
+
 export function getOrganization(
   client: ApiClient,
   id: string,
 ): OrganizationRoute {
-  // @ts-expect-error - Eden Treaty Proxy allows dynamic property access
-  return client.organizations[id];
+  return (client.organizations as unknown as DynamicIndex<OrganizationRoute>)[
+    id
+  ] as OrganizationRoute;
 }
 
 export function getChannel(client: ApiClient, id: string): ChannelRoute {
-  // @ts-expect-error - Eden Treaty Proxy allows dynamic property access
-  return client.channels[id];
+  return (client.channels as unknown as DynamicIndex<ChannelRoute>)[
+    id
+  ] as ChannelRoute;
 }
 
 export function getMessage(client: ApiClient, id: string): MessagesRoute {
-  // @ts-expect-error - Eden Treaty Proxy allows dynamic property access
-  return client.messages[id];
+  return (client.messages as unknown as DynamicIndex<MessagesRoute>)[
+    id
+  ] as MessagesRoute;
 }
 
 export function getVote(client: ApiClient, id: string): VoteRoute {
-  // @ts-expect-error - Eden Treaty Proxy allows dynamic property access
-  return client.votes[id];
-}
-
-export function getTask(client: ApiClient, id: string): TaskRoute {
-  // @ts-expect-error - Eden Treaty Proxy allows dynamic property access
-  return client.tasks[id];
+  return (client.votes as unknown as DynamicIndex<VoteRoute>)[id] as VoteRoute;
 }
 
 export function getFile(client: ApiClient, id: string): FileRoute {
-  // @ts-expect-error - Eden Treaty Proxy allows dynamic property access
-  return client.files[id];
+  return (client.files as unknown as DynamicIndex<FileRoute>)[id] as FileRoute;
 }
diff --git a/packages/api-client/src/types.ts b/packages/api-client/src/types.ts
index 4d6942d..801e9e3 100644
--- a/packages/api-client/src/types.ts
+++ b/packages/api-client/src/types.ts
@@ -1,5 +1,14 @@
 // Type definitions for API responses
 
+/**
+ * 標準的なAPIエラーレスポンス
+ */
+export interface ApiErrorResponse {
+  message: string;
+  code?: string;
+  details?: unknown;
+}
+
 export interface User {
   id: string;
   email?: string | null;