Adds a persisted Chat/Research mode selector, durable progress timeline, cancellation, navigation restoration, and a rendered/downloadable Markdown report experience.
Research state loads in parallel with thread state, and unsupported attachments are rejected instead of silently ignored.
Validation: 2 focused web tests, Biome, and diff checks.
## Summary
- Add custom assistant-message rendering for Nova connector tool results
- Show integration-style setup/status cards with icon, status pill, setup steps, docs links, copy buttons, and key reveal/generate action
- Keep plugin API keys client-side only by calling the existing `/v3/auth/key?client=...` endpoint from the UI
- Add Nova empty-state suggestions for Cursor setup and active plugins
