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/explore/HANDOFF.md

@@ -0,0 +1,79 @@
+# Explore — handoff
+
+## Summary
+
+Discover public rooms and featured communities on Matrix homeservers. Browse by server, search public room directory, and preview/join rooms.
+
+## User-facing behavior
+
+- Explore tab in sidebar
+- Pick server / homeserver directory
+- Featured rooms list
+- Search public rooms on server
+- Join room from preview
+
+## Architecture
+
+```
+client/explore/
+  Explore.tsx — main explore page
+  Server.tsx — server-specific directory
+  Featured.tsx — featured rooms
+  PublicRooms via matrix directory API
+```
+
+## Key files
+
+| Path | Role |
+|------|------|
+| `cinny/src/app/pages/client/explore/Explore.tsx` | Explore hub |
+| `cinny/src/app/pages/client/explore/Server.tsx` | Server directory |
+| `cinny/src/app/pages/client/explore/Featured.tsx` | Featured list |
+| `cinny/src/app/pages/client/sidebar/ExploreTab.tsx` | Sidebar entry |
+| `cinny/src/app/hooks/useRoomDirectoryVisibility.ts` | Directory visibility |
+
+## Data model
+
+Public room directory from homeserver HTTP API (ephemeral).
+
+## Dependencies
+
+- Homeserver `/publicRooms` or sliding sync directory
+- `matrix-js-sdk` room directory helpers
+
+## Integration points
+
+- **routing-and-navigation**: `EXPLORE_PATH`, `_SERVER_PATH`, `_FEATURED_PATH`
+- **join-before-navigate**: may route here before join
+
+## Testing
+
+### Manual
+
+1. Open explore; list loads for default server.
+2. Switch server; directory refreshes.
+3. Join public room from explore.
+4. Server without public directory — error handling.
+
+### Automated
+
+- None
+
+## Known issues & gotchas
+
+- Public directory disabled on many private homeservers
+- Featured rooms may be config-dependent (`clientConfig`)
+
+## Future work
+
+- Federated universal search
+- Room preview with topic/avatar before join
+
+## Related docs
+
+- [routing-and-navigation HANDOFF](../routing-and-navigation/HANDOFF.md)
+- [join-before-navigate HANDOFF](../join-before-navigate/HANDOFF.md)
+
+## Add your extra things here
+
+- `getExploreFeaturedPath` in pathUtils