- routes.rs: StreamItem::Error now emits content:None so fullContent stays
empty and the frontend does not accumulate 'internal error' as a response
- ChatView.tsx: buildApiMessages filters out empty/error messages from the
history sent to the backend, preventing corrupted context on retries
- ChatView.tsx: assistant INSERT is guarded — skipped when fullContent is
empty, so a failed stream leaves no empty assistant row in the DB