fix(chat): suppress Unauthorized unhandled rejection from optimistic init

[Movm]

Summary

Stale cached userId in localStorage triggers eager threadListAdapter.initialize() on /login before useAuth clears the session. The resulting 401 from POST /api/chat-service/threads surfaces as an unhandled rejection captured by Sentry — even though onUnauthorized() in chatConfig already handles the redirect side effect.

This extends the existing "Thread not found" suppression in GrueneratorChatProvider to also drop "Unauthorized" rejections — same root-cause class (assistant-ui optimisticUpdate paths we can't intercept via onClick).

Root cause

1. Zustand authStore rehydrates synchronously from localStorage (15-min TTL cache).
2. On /login, cached userId is non-empty even though the backend session is invalid.
3. The if (!userId) return children guard at GrueneratorChatProvider.tsx:421 passes; runtime mounts.
4. assistant-ui's optimisticUpdate.initialize eagerly calls threadListAdapter.initialize(), which POSTs /api/chat-service/threads.
5. Backend returns 401, request() in ChatContext.tsx:33 calls onUnauthorized() then throw new Error('Unauthorized').
6. The rejection is re-emitted asynchronously by assistant-ui's optimistic rollback, bypassing history.load's try/catch — Sentry captures it.

The redirect side effect already happens before the throw, so suppressing the unhandled rejection is purely about Sentry noise.

GlitchTip issue: GRUENERATOR-46 (494 events).

Test plan

• Verify normal chat thread creation still works for authenticated users
• Verify "Thread not found" suppression still works (delete a thread)
• Verify a 401 on /login no longer hits Sentry but onUnauthorized redirect still fires when the user is on a protected page