refactor: use TanStack Query cache as single source of truth for chat messages

[saraichinwag]

Summary

Eliminates dual-state race conditions in ChatSidebar by using TanStack Query cache as the single source of truth for chat messages.

Changes

ChatSidebar.jsx:

• Removed useState for messages — reads directly from useChatSession() query data
• Optimistic updates use queryClient.setQueryData instead of setMessages
• New sessions use a temporary pendingUserMessage state until the session is created and cache is seeded
• Removed useEffect syncing sessionQuery.data.conversation → setMessages
• Removed useEffect for sessionQuery.error (handled inline)

useChatTurn.js:

• processToCompletion now accepts queryClient instead of onNewMessages callback
• continueTurn updates query cache directly via appendMessagesToCache helper
• No more stale closure issues from callback-based state updates

queries/chat.js: No changes needed — staleTime: Infinity already prevents cache overwrites.

Why This Works

• Messages come from ONE place: the query cache for ['chat-session', sessionId]
• staleTime: Infinity ensures our optimistic updates aren't overwritten by refetches
• Server responses seed the cache directly, so there's no sync delay
• Turn-by-turn polling appends to cache atomically

Fixes #210
Fixes #4
Fixes #5