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/matrix-client/HANDOFF.md

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+# Matrix client — handoff
+
+## Summary
+
+The Matrix client layer wraps `matrix-js-sdk`: client creation, IndexedDB crypto/sync stores, start/stop sync, logout, and React context providers. `ClientRoot` is the authenticated app bootstrapper.
+
+## User-facing behavior
+
+- Splash screen ("Heating up") while client initializes
+- Sync status reflected in UI (`SyncStatus`, splash)
+- Logout from settings or forced server logout
+- Mobile: visibility-based sync restart
+
+## Architecture
+
+```
+ClientRoot.tsx
+  → getCurrentSession() → initClient(session)
+  → startClient(mx)
+  → Providers: MatrixClient, Capabilities, MediaConfig, Call, PluginLoader
+initMatrix.ts
+  → createClient + IndexedDBStore + IndexedDBCryptoStore
+  → secretStorageKeys / cryptoCallbacks
+```
+
+## Key files
+
+| Path | Role |
+|------|------|
+| `cinny/src/client/initMatrix.ts` | createClient, start/stop, logout, cache clear |
+| `cinny/src/client/secretStorageKeys.ts` | SSSS / secret storage callbacks |
+| `cinny/src/app/pages/client/ClientRoot.tsx` | Bootstrap, providers, title bar |
+| `cinny/src/app/hooks/useMatrixClient.ts` | React context for `MatrixClient` |
+| `cinny/src/app/hooks/useSyncState.ts` | Sync state subscription |
+| `cinny/src/app/pages/client/SyncStatus.tsx` | Sync indicator UI |
+| `cinny/src/app/components/splash-screen/SplashScreen.tsx` | Loading shell |
+
+## Data model
+
+| Store | Backend | Purpose |
+|-------|---------|---------|
+| IndexedDB sync store | matrix-js-sdk | Room timeline cache |
+| IndexedDB crypto store | matrix-js-sdk | Olm/Megolm keys |
+| Session | localStorage | Auth credentials |
+
+## Dependencies
+
+- `matrix-js-sdk`
+- IndexedDB (browser/Electron)
+
+## Integration points
+
+- **authentication**: session → initClient
+- **state-management**: sync events feed atoms
+- **background-notifications**: pushers registered after sync
+- **e2e-and-devices**: crypto store, verification
+- **plugins**: Matrix client exposed to plugin API
+
+## Testing
+
+### Manual
+
+1. Fresh login; verify sync completes (timeline loads).
+2. Reload app; session restores without re-login.
+3. Logout; IndexedDB cleared appropriately.
+4. Airplane mode → sync error → recovery on reconnect.
+
+### Automated
+
+- None
+
+## Known issues & gotchas
+
+- `handlingKeyConflict` flag in initMatrix for crypto key conflicts
+- `deleteAllMatrixDatabases` on certain logout/clear paths — destructive
+- Mobile `useVisibilitySync` restarts sync on app resume (Tauri)
+- `clearCacheAndReload` utility for support scenarios
+
+## Future work
+
+- Lazy sync / sliding sync if migrating MSC
+- Better sync error UX with retry backoff
+
+## Related docs
+
+- [authentication HANDOFF](../authentication/HANDOFF.md)
+- [state-management HANDOFF](../state-management/HANDOFF.md)
+- [e2e-and-devices HANDOFF](../e2e-and-devices/HANDOFF.md)
+
+## Add your extra things here
+
+- `logoutClient`, `clearLoginData`, `clearCacheAndReload` exported from initMatrix.ts
+- `HttpApiEvent.SessionLoggedOut` listener in ClientRoot clears everything