feat: add inbox page for GitHub notifications

[stylessh]

Summary

• Adds a split-view Inbox page (Linear-style) with a notification list sidebar and a content preview pane that renders actual PR/Issue detail pages inline
• Notifications show state-aware icons/colors (open/closed/merged), participant avatar stacks (author, assignees, reviewers, commenters), and support mark-as-read, mark-as-done, and archive actions with optimistic UI updates
• Prefetches the next notification's detail data on selection for instant navigation
• Fixes closed PR merge status display across PR detail header and user activity feed — branch refs and "wants to merge" text no longer show for closed-unmerged PRs

Changes

• New: apps/dashboard/src/components/inbox/inbox-page.tsx — full inbox UI with InboxPage, InboxSidebar, InboxRow, InboxPreview, AvatarStack
• New: apps/dashboard/src/routes/_protected/inbox.tsx — route definition
• New: packages/icons/src/archive-down-icon.tsx — custom archive icon
• Modified: github.functions.ts — getNotifications, markNotificationRead, markNotificationDone, markAllNotificationsRead server functions; fetches participants (commenters, reviewers) and subject state
• Modified: github.types.ts — NotificationItem, NotificationSubject, NotificationParticipant types
• Modified: github.query.ts — notification query options
• Modified: pull-detail-page.tsx / issue-detail-page.tsx — extracted reusable PullDetailContent / IssueDetailContent components
• Modified: pull-detail-header.tsx — hide merge info for closed-unmerged PRs, show "merged into" for merged PRs
• Modified: user-activity-feed.tsx — hide branch refs for closed-unmerged PRs
• Modified: topbar + mobile nav — added Inbox nav item

Test plan

• Navigate to /inbox and verify notifications load with correct state icons/colors
• Click a PR notification — verify PR detail renders in preview pane
• Click an Issue notification — verify Issue detail renders in preview pane
• Verify avatar stack shows participants (hover for login tooltips)
• Archive a notification — verify it animates out
• Mark as read — verify unread dot disappears without removing the item
• Switch between Unread/All filters
• Use ellipsis menu to mark all as read / archive all read
• Verify closed-unmerged PRs don't show "wants to merge" or branch refs
• Verify merged PRs show "merged into" text

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Status	Name	Latest Commit	Updated (UTC)
✅ Deployment successful! View logs	diffkit	cd5abc9	Apr 15 2026, 12:38 AM