feat: upgrade toolchain and soften sync recovery after network blips

Bump Vite 8, matrix-js-sdk, and related deps; fix immer/vanilla-extract imports for new majors. Replace blocking sync error dialog with background recovery on wake/offline, and disable Vite auto-open in dev. Add feature handoff documentation index.

docs/handoff/features/developer-tools/HANDOFF.md

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+# Developer tools — handoff
+
+## Summary
+
+Power-user tools for debugging Matrix state: view/edit account data, send custom room events, and inspect raw JSON. Available in user settings and room/space common-settings.
+
+## User-facing behavior
+
+- **Settings → Developer tools**: account data viewer
+- **Room settings → Developer tools**: send state events, room account data
+- Raw JSON editors with validation warnings
+
+## Architecture
+
+```
+settings/developer-tools/
+  DevelopTools.tsx, AccountData.tsx
+common-settings/developer-tools/
+  DevelopTools.tsx, StateEventEditor.tsx, SendRoomEvent.tsx
+```
+
+## Key files
+
+| Path | Role |
+|------|------|
+| `cinny/src/app/features/settings/developer-tools/DevelopTools.tsx` | User-level dev tools |
+| `cinny/src/app/features/settings/developer-tools/AccountData.tsx` | Account data viewer |
+| `cinny/src/app/features/common-settings/developer-tools/DevelopTools.tsx` | Room dev tools entry |
+| `cinny/src/app/features/common-settings/developer-tools/StateEventEditor.tsx` | Edit state events |
+| `cinny/src/app/features/common-settings/developer-tools/SendRoomEvent.tsx` | Send events |
+
+## Data model
+
+Direct read/write to Matrix account data and state events — **production danger**.
+
+## Dependencies
+
+- matrix-js-sdk low-level APIs
+
+## Integration points
+
+- **user-settings**: developer tab
+- **common-settings**: per-room tools
+
+## Testing
+
+### Manual
+
+1. View account data keys on staging account.
+2. Send harmless custom state event in test room.
+3. Verify permission denied without power level.
+
+### Automated
+
+- None
+
+## Known issues & gotchas
+
+- No guard rails — can break rooms with bad state events
+- Should be hidden or disabled in production builds for normal users (currently settings-gated only)
+- Typo `DevelopTools` vs `DeveloperTools` in filenames
+
+## Future work
+
+- Feature flag via client config
+- Event diff viewer
+
+## Related docs
+
+- [user-settings HANDOFF](../user-settings/HANDOFF.md)
+- [common-settings HANDOFF](../common-settings/HANDOFF.md)
+
+## Add your extra things here
+
+- Only use on test homeservers — document internal MSC event types here when experimenting