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/home-and-sidebar/HANDOFF.md

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+# Home & sidebar — handoff
+
+## Summary
+
+The **Home** panel lists joined rooms (with sub-room nesting), and the **sidebar** switches between Home, DMs, Explore, Inbox, Search, Create, and Settings tabs. Room ordering, categories, and unread badges are rendered here.
+
+## User-facing behavior
+
+- Home: hierarchical room list with categories, drag reorder, unread/highlight states
+- Sidebar tabs with Paarrot logo states (normal/unread/highlight) on Home
+- Space tabs when inside a space context
+- Threads category on home (`HomeThreadsCategory`)
+- Sync status in client chrome
+
+## Architecture
+
+```
+SidebarNav → tab routes (HomeTab, DirectTab, ExploreTab, ...)
+Home.tsx
+  → useHomeRooms.ts (room list + sub-room flattening)
+  → RoomNavItem / RoomNavCategoryButton / UnjoinedSubRoomItem
+DirectTab.tsx → useDirectRooms.ts
+```
+
+## Key files
+
+| Path | Role |
+|------|------|
+| `cinny/src/app/pages/client/home/Home.tsx` | Home room list UI |
+| `cinny/src/app/pages/client/home/useHomeRooms.ts` | Room list logic + Paarrot room kind |
+| `cinny/src/app/pages/client/SidebarNav.tsx` | Sidebar tab bar |
+| `cinny/src/app/pages/client/sidebar/HomeTab.tsx` | Home tab |
+| `cinny/src/app/pages/client/sidebar/DirectTab.tsx` | DMs tab + favicon logos |
+| `cinny/src/app/pages/client/sidebar/InboxTab.tsx` | Inbox tab |
+| `cinny/src/app/pages/client/sidebar/ExploreTab.tsx` | Explore tab |
+| `cinny/src/app/pages/client/sidebar/SearchTab.tsx` | Search tab |
+| `cinny/src/app/pages/client/sidebar/SettingsTab.tsx` | Settings tab |
+| `cinny/src/app/pages/client/sidebar/SpaceTabs.tsx` | Space context tabs |
+| `cinny/src/app/pages/client/home/HomeThreadsCategory.tsx` | Threads grouping |
+| `cinny/src/app/hooks/useRoomListReorder.ts` | DnD reorder |
+| `cinny/src/app/hooks/useSidebarItems.ts` | Sidebar item model |
+
+## Data model
+
+| Source | Data |
+|--------|------|
+| `allRoomsAtom` | Joined rooms |
+| `roomToUnreadAtom` | Badge state |
+| `mDirectAtom` | DM room mapping |
+| `im.paarrot.sub_rooms` | Nested children (see sub-rooms) |
+| `im.paarrot.room.kind` | Room classification |
+| localStorage | Category collapse, room order |
+
+## Dependencies
+
+- Jotai room list atoms
+- `matrix-js-sdk` room summaries
+
+## Integration points
+
+- **sub-rooms**: tree flattening in Home.tsx
+- **room-nav**: nav item components
+- **routing-and-navigation**: tab routes
+- **background-notifications**: unread → favicon in DirectTab
+
+## Testing
+
+### Manual
+
+1. Verify room order drag persists.
+2. Sub-room nesting indent and unjoined state.
+3. Unread badge on sidebar tab.
+4. Category collapse/expand.
+5. Switch tabs; active state correct.
+
+### Automated
+
+- None
+
+## Known issues & gotchas
+
+- `useHomeRooms` filters top-level vs sub-room IDs — easy to break tree
+- DirectTab uses custom Paarrot SVGs for badge states
+- Mobile hides sidebar in favor of bottom/full-page nav
+
+## Future work
+
+- Unified sidebar component between home and space contexts
+- Virtualize long room lists
+
+## Related docs
+
+- [sub-rooms HANDOFF](../sub-rooms/HANDOFF.md)
+- [room-nav HANDOFF](../room-nav/HANDOFF.md)
+- [routing-and-navigation HANDOFF](../routing-and-navigation/HANDOFF.md)
+
+## Add your extra things here
+
+- `RoomNavCategoryButton` for collapsible sections
+- `useClosedNavCategoriesAtom` for collapsed state