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/electron-shell/HANDOFF.md

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+# Electron desktop shell — handoff
+
+## Summary
+
+The desktop app wraps the Cinny/Paarrot web UI in Electron: custom title bar, system tray, deep links (`paarrot://`), auto-updates, plugin filesystem access, screen capture for calls, and the local HTTP API server.
+
+## User-facing behavior
+
+- Frameless/custom **title bar** with window controls (Windows/Linux)
+- Minimize to **tray**; close may minimize instead of quit (platform config)
+- `paarrot://` URL protocol for room links
+- Auto-update checks (electron-updater)
+- Plugins read/write under `%APPDATA%\paarrot\`
+
+## Architecture
+
+```
+electron/main.js (main process)
+  → BrowserWindow → Vite dev server or built cinny dist
+  → IPC: plugins, screen capture, clipboard, updates
+  → PaarrotAPIServer
+  → Tray + protocol handler paarrot://
+preload / contextBridge → window.electron in renderer
+```
+
+## Key files
+
+| Path | Role |
+|------|------|
+| `electron/main.js` | Main process entry |
+| `electron/api-server.js` | Local HTTP API |
+| `electron/preload.js` (if exists) | IPC bridge |
+| `electron-builder.json5` | Packaging config |
+| `package.json` (root) | `npm run dev`, build scripts |
+| `dev-app-update.yml` | Update channel config |
+| `cinny/src/app/components/title-bar/TitleBar.tsx` | Custom title bar UI |
+| `build-resources/` | Linux after-install, icons |
+
+## Data model
+
+| Store | Tool | Contents |
+|-------|------|----------|
+| `electron-store` | main.js | Window bounds, user prefs |
+| UserData | Electron | plugins, caches |
+
+Protocol: `PROTOCOL_SCHEME = 'paarrot'` in main.js
+
+## Dependencies
+
+- `electron`, `electron-updater`, `electron-store`
+- `electron-builder` for CI releases
+- Vite dev server port **38347**; static server **44548** in dev
+
+## Integration points
+
+- **local-api**: started with main window
+- **Plugins**: filesystem paths under `paarrot` app name
+- **Voice calls**: `desktopCapturer` for screen share
+- **Deep links**: pending deep link queue until client ready
+
+## Testing
+
+### Manual
+
+1. `npm run dev` — window loads, title bar draggable.
+2. Register `paarrot://` link; opens correct room.
+3. Tray icon present; restore from tray.
+4. Build installer; auto-update check (staging).
+5. Install plugin to AppData path; loader finds it.
+
+### Automated
+
+- CI: `.gitea/workflows/build.yml`
+
+## Known issues & gotchas
+
+- `isDev` detection: `NODE_ENV` or `!app.isPackaged`
+- Protocol registration differs Windows vs Linux (`after-install.sh`)
+- Single instance lock for deep links — see `pendingDeepLink` in main.js
+- Title bar hit-testing CSS sensitive on Windows 11
+
+## Future work
+
+- macOS menu bar parity
+- Code signing documentation for Windows
+- Consolidate magic ports into shared config
+
+## Related docs
+
+- [local-api HANDOFF](../local-api/HANDOFF.md)
+- Root [README.md](../../../../README.md) — build instructions
+
+## Add your extra things here
+
+- Dev URLs: `VITE_DEV_SERVER = http://localhost:38347`, `PORT = 44548`
+- App userData folder name: `paarrot` (matches plugin paths in README)