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/client-chrome/HANDOFF.md

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+# Client chrome — handoff
+
+## Summary
+
+App shell UI outside core chat: custom title bar (Electron), splash screen, welcome page, account switcher, update notifications, sync status, spec version warnings, and feature capability checks.
+
+## User-facing behavior
+
+- **Title bar**: drag region, window controls, room title, account menu (desktop)
+- **Splash**: loading spinner during client init
+- **Welcome page**: first-run / empty state hints
+- **Account switcher**: switch between logged-in accounts (if multi-session)
+- **Update notification**: new version available (electron-updater)
+- **Feature check**: block unsupported browsers
+- **Spec versions**: warn on old homeserver
+
+## Architecture
+
+```
+ClientRoot → TitleBar + AccountSwitcher + children
+SplashScreen during matrix init
+WelcomePage on empty states
+UpdateNotification component
+FeatureCheck wraps App
+SpecVersions in ClientRoot
+```
+
+## Key files
+
+| Path | Role |
+|------|------|
+| `cinny/src/app/components/title-bar/TitleBar.tsx` | Desktop title bar |
+| `cinny/src/app/components/splash-screen/SplashScreen.tsx` | Splash |
+| `cinny/src/app/pages/client/WelcomePage.tsx` | Welcome / download links |
+| `cinny/src/app/components/account-switcher/AccountSwitcher.tsx` | Multi-account |
+| `cinny/src/app/components/update-notification/UpdateNotification.tsx` | Auto-update UI |
+| `cinny/src/app/pages/FeatureCheck.tsx` | Browser capability gate |
+| `cinny/src/app/pages/client/SpecVersions.tsx` | HS version warning |
+| `cinny/src/app/pages/client/SyncStatus.tsx` | Sync indicator |
+| `cinny/src/app/pages/ConfigConfig.tsx` | Config load error UI |
+| `cinny/src/app/components/ClientConfigLoader.tsx` | Load config.json |
+
+## Data model
+
+| Key | Storage |
+|-----|---------|
+| Sessions | multi-account in sessions.ts |
+| electron-store | window bounds (main process) |
+
+## Dependencies
+
+- Electron IPC for window controls
+- `electron-updater` for updates
+- `clientConfig` JSON (homeserver default, hash router, etc.)
+
+## Integration points
+
+- **electron-shell**: title bar IPC
+- **matrix-client**: splash until sync ready
+- **authentication**: account switcher sessions
+
+## Testing
+
+### Manual
+
+1. Desktop: minimize/maximize/close via title bar.
+2. Slow network: splash shows until ready.
+3. Trigger update check in dev (if configured).
+4. Unsupported browser: FeatureCheck message.
+
+### Automated
+
+- None
+
+## Known issues & gotchas
+
+- Title bar only on Electron — web uses browser chrome
+- Welcome page links point to GitHub releases — verify URLs match your distribution (Gitea vs GitHub)
+- `FeatureCheck` may block older WebViews on Android
+
+## Future work
+
+- Unified status bar (sync + update + call status)
+- macOS traffic lights styling
+
+## Related docs
+
+- [electron-shell HANDOFF](../electron-shell/HANDOFF.md)
+- [matrix-client HANDOFF](../matrix-client/HANDOFF.md)
+- [authentication HANDOFF](../authentication/HANDOFF.md)
+
+## Add your extra things here
+
+- Title bar shows `Paarrot` prefix + dynamic room/space title suffix
+- `ClientConfigLoader` reads public config — document config.json schema in client config handoff if split later