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/routing-and-navigation/HANDOFF.md

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+# Routing & navigation — handoff
+
+## Summary
+
+React Router drives all app navigation: auth routes, client shell, home/direct/space/inbox/explore paths, room views, lobby/forum feeds, and modal overlays (settings, search, create). Supports hash router mode via client config.
+
+## User-facing behavior
+
+- URL reflects current room, space, inbox tab, explore server
+- Mobile vs desktop route layouts (`MobileFriendly` wrappers)
+- Deep links and `matrix.to` style joins via join route
+- After-login redirect preserves intended destination
+- View transitions on supported platforms
+
+## Architecture
+
+```
+App.tsx → createRouter(clientConfig, screenSize)
+  → Auth routes | ClientLayout + SidebarNav + Outlet
+paths.ts — route constants
+pathUtils.ts — path builders (getHomeRoomPath, getSpaceDefaultPath, ...)
+Router.tsx — route tree + loaders/guards
+```
+
+## Key files
+
+| Path | Role |
+|------|------|
+| `cinny/src/app/pages/Router.tsx` | Full route tree |
+| `cinny/src/app/pages/paths.ts` | Path constants |
+| `cinny/src/app/pages/pathUtils.ts` | Path builder utilities |
+| `cinny/src/app/pages/App.tsx` | RouterProvider setup |
+| `cinny/src/app/pages/client/ClientLayout.tsx` | Authenticated layout shell |
+| `cinny/src/app/pages/client/SidebarNav.tsx` | Left sidebar tabs |
+| `cinny/src/app/pages/MobileFriendly.tsx` | Mobile nav adapters |
+| `cinny/src/app/hooks/useRoomNavigate.ts` | Imperative room navigation |
+| `cinny/src/app/hooks/useNavToActivePathMapper.ts` | Restore last path |
+| `cinny/src/app/pages/afterLoginRedirectPath.ts` | Login redirect |
+| `cinny/src/app/components/AnimatedOutlet.tsx` | Route transitions |
+
+## Data model
+
+| Key | Purpose |
+|-----|---------|
+| `navToActivePath` atom | Per-context last URL |
+| URL path params | roomId, spaceId, eventId |
+
+## Dependencies
+
+- `react-router-dom` v6
+- `clientConfig.hashRouter` for Electron/file protocol
+
+## Integration points
+
+- **forum**: `ForumAwareSpaceLayout`, `ForumFeedPage` routes
+- **lobby-forums**: `_LOBBY_PATH`, `Lobby` component
+- **join-before-navigate**: `_JOIN_PATH` route
+- **local-api**: `setPaarrotNavigate` uses same path utils
+- **settings/room-settings**: overlay renderers on client routes
+
+## Testing
+
+### Manual
+
+1. Navigate home → room → back; URL updates correctly.
+2. Open space; forum vs lobby path per room type.
+3. Hash router mode (if enabled in config).
+4. Mobile breakpoints: sidebar vs full-page nav.
+5. Refresh on deep room URL — lands in same room.
+
+### Automated
+
+- None
+
+## Known issues & gotchas
+
+- `getSpaceDefaultPath` branches forum vs lobby — must stay in sync with forum handoff
+- Hash router needed for some Electron `file://` deployments
+- Many path helpers — prefer using `pathUtils` over string concat
+
+## Future work
+
+- Route-level code splitting audit
+- Unified join/link handler
+
+## Related docs
+
+- [forum HANDOFF](../forum/HANDOFF.md)
+- [lobby-forums HANDOFF](../lobby-forums/HANDOFF.md)
+- [join-before-navigate HANDOFF](../join-before-navigate/HANDOFF.md)
+- [home-and-sidebar HANDOFF](../home-and-sidebar/HANDOFF.md)
+
+## Add your extra things here
+
+- Key paths: `HOME_PATH`, `DIRECT_PATH`, `SPACE_PATH`, `INBOX_PATH`, `EXPLORE_PATH`, `_FEED_PATH`, `_LOBBY_PATH`
+- `getAppPathFromHref` for parsing external links