- tweetImages and imageUrl set to null instead of undefined when empty
(Automerge rejects undefined as invalid JSON)
- Updated ThreadData schema to allow null for optional fields
- This was the root cause of the 404 on image generation: saveDraft
threw on undefined, so the thread never synced to the server, and
the server-side route returned "Thread not found"
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>