Port high-value features from work/m4-llm branch into develop's
security-hardened codebase:
- Separate LLM vs persistence error handling in useChat (shows
assistant response even when save fails)
- Add structured error context logging with errorType, messagePreview,
messageCount fields for debugging
- Enforce state invariant in useChatOverlay: cannot be minimized when
closed
- Add onStorageError callback with user-friendly messages and
per-error-type deduplication
- Add error logging to Chat imperative handle methods
- Create Chat.test.tsx with loadConversation failure mode tests
Skipped from work/m4-llm (superseded by develop):
- AbortSignal timeout (develop has centralized client timeout)
- Custom toast system (duplicates @mosaic/ui)
- ErrorBoundary (develop has its own)
- WebSocket typed events (develop's ref-based pattern is superior)
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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