- Replace single selectedNote with tabs state in App (open/close/switch)
- Tab bar above editor: title + x close button, active tab highlighted
- Clicking note opens new tab or switches to existing
- Closing active tab switches to adjacent tab
- Fix race condition: editor re-creates when tab content loads async
- Add Playwright test verifying multi-tab behavior
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>