Move `pendingNewChat` navigation logic from useEffect to event handler

[MODSetter]

Description

In LayoutDataProvider.tsx, the "new chat" navigation is split across an event handler and a useEffect via a pendingNewChat boolean flag. The handler sets the flag (line 553), and a separate effect watches it plus params?.chat_id to complete the navigation (lines 261-267). This is the classic "event → setState → effect" anti-pattern that React docs warn against.

Vercel React Best Practices Rule: rerender-move-effect-to-event (5.7)

File to change

• surfsense_web/components/layout/providers/LayoutDataProvider.tsx

What to do

1. Remove the pendingNewChat state and its useEffect (lines 259-268).

2. Refactor handleNewChat (lines 544-558) to handle the full navigation in one flow:

const handleNewChat = useCallback(() => {
  const isOutOfSync = currentThreadState.id !== null && !params?.chat_id;
  if (isOutOfSync) {
    resetCurrentThread();
    router.replace(`/dashboard/${searchSpaceId}/new-chat/${currentThreadState.id}`);
    // Allow router to sync, then navigate to fresh new-chat
    setTimeout(() => {
      router.push(`/dashboard/${searchSpaceId}/new-chat`);
    }, 0);
  } else {
    router.push(`/dashboard/${searchSpaceId}/new-chat`);
  }
}, [router, searchSpaceId, currentThreadState.id, params?.chat_id, resetCurrentThread]);

3. Remove pendingNewChat and setPendingNewChat from the component entirely.

Acceptance criteria

• "New Chat" button still works from any state (fresh page, existing chat, out-of-sync URL)
• No pendingNewChat state or associated useEffect exists
• Navigation flow is traceable in a single function

[JoeMakuta]

I can work on this @MODSetter