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/background-notifications/HANDOFF.md

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+# Background notifications — handoff
+
+## Summary
+
+Paarrot keeps users notified of Matrix activity when the app is in the background: desktop tray/favicon badges, system notifications (Electron/Tauri/Android), and Android **UnifiedPush** for reliable mobile push without Google FCM dependency.
+
+## User-facing behavior
+
+- Unread counts update favicon (Paarrot / unread / highlight variants)
+- System notifications for mentions/DMs per notification settings
+- Android: requires UnifiedPush distributor (e.g. ntfy); user prompted if missing
+- Foreground/background state affects sync strategy
+
+## Architecture
+
+```
+ClientNonUIFeatures.tsx
+  → FaviconUpdater (roomToUnread atom)
+  → notification listeners (tauri.ts sendNotification)
+  → initPaarrotAPI + background sync lifecycle
+backgroundSync.ts
+  → UnifiedPush registration, pusher setup
+  → setAppForegroundState / startBackgroundSync / stopBackgroundSync
+tauri.ts / electron main
+  → OS notification APIs, Android small icon ic_stat_paarrot
+```
+
+## Key files
+
+| Path | Role |
+|------|------|
+| `cinny/src/app/pages/client/ClientNonUIFeatures.tsx` | Favicon, notifications, API init, background sync hooks |
+| `cinny/src/app/utils/backgroundSync.ts` | Pushers, UnifiedPush, foreground state |
+| `cinny/src/app/utils/tauri.ts` | Tauri/Capacitor notification helpers |
+| `cinny/src/app/features/settings/notifications/SystemNotification.tsx` | Permission UX |
+| `electron/main.js` | Tray, notification IPC (if applicable) |
+| `src-tauri/src/main.rs` | Tauri Android shell |
+
+## Data model
+
+| Key | Storage | Purpose |
+|-----|---------|---------|
+| `paarrot.unifiedpush.*` | localStorage prefix | UP endpoint state |
+| `com.paarrot.app.android` | Pusher app id base | Android pushers |
+| Matrix pusher | Homeserver | HTTP push gateway |
+
+## Dependencies
+
+- UnifiedPush distributor app on Android
+- Notification permission (browser/OS)
+- Homeserver pusher support
+- `matrix-js-sdk` sync
+
+## Integration points
+
+- **Settings → Notifications**: rules + system permission
+- **DirectTab / sidebar**: custom Paarrot SVG favicon states
+- **Deep links**: notification tap → `setupNotificationTapListener`
+
+## Testing
+
+### Manual
+
+1. Desktop: background app; send mention; verify system notification.
+2. Verify favicon switches unread/highlight with counts.
+3. Android: install ntfy; verify push when app killed.
+4. Toggle notification settings; verify respect mute rules.
+
+### Automated
+
+- None
+
+## Known issues & gotchas
+
+- UnifiedPush user education string in `backgroundSync.ts` if no distributor
+- Multiple runtimes: `isTauri`, `isElectron`, `isCapacitorNative` branch logic
+- Pusher registration races on fast login/logout
+- Android small icon: `ic_stat_paarrot` must exist in Android resources
+
+## Future work
+
+- iOS push strategy
+- Notification action buttons (reply, mark read)
+- Per-room notification channels on Android
+
+## Related docs
+
+- Matrix push gateway docs (upstream)
+
+## Add your extra things here
+
+- Constants: `PUSHER_APP_ID_BASE`, `PUSHER_STORAGE_PREFIX` in `backgroundSync.ts`
+- Logo assets: `paarrot.svg`, `paarrot-unread.svg`, `paarrot-highlight.svg` in `public/res/svg/`