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/sub-rooms/HANDOFF.md

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+# Sub-rooms — handoff
+
+## Summary
+
+Sub-rooms let a parent room own an ordered list of child rooms, displayed nested in the home channel list and manageable in room settings. Hierarchy is stored in Matrix room state (`im.paarrot.sub_rooms`), not only `m.space.child`, so channels can nest under a room without full space semantics.
+
+## User-facing behavior
+
+- Nested channels under a room in **Home** sidebar (indented tree)
+- **Add Sub-Room** from room nav context menu (requires power level on `im.paarrot.sub_rooms`)
+- **Room settings → Sub-rooms** to reorder/remove children
+- Creating a sub-room from parent updates parent's `children` array
+- Unjoined sub-rooms show as `UnjoinedSubRoomItem` with join affordance
+
+## Architecture
+
+```
+Room state: im.paarrot.sub_rooms { children: string[] }
+  → useRoomSubRooms hook
+    → Home.tsx flatten tree (addRoomAndSubRooms)
+    → RoomNavItem "Add Sub-Room" → CreateRoomModal
+    → SubRooms.tsx settings editor
+    → SpaceHierarchy.tsx (lobby) reads same state
+```
+
+## Key files
+
+| Path | Role |
+|------|------|
+| `cinny/src/app/hooks/useRoomSubRooms.ts` | Hook + `PaarrotSubRoomsContent` type |
+| `cinny/src/types/matrix/room.ts` | `StateEvent.PaarrotSubRooms` |
+| `cinny/src/app/pages/client/home/Home.tsx` | Renders nested list |
+| `cinny/src/app/features/room-nav/RoomNavItem.tsx` | Add sub-room action |
+| `cinny/src/app/features/room-nav/UnjoinedSubRoomItem.tsx` | Unjoined child UI |
+| `cinny/src/app/features/room-settings/sub-rooms/SubRooms.tsx` | Settings page |
+| `cinny/src/app/features/create-room/CreateRoomModal.tsx` | Appends new room to parent state |
+| `cinny/src/app/features/lobby/SpaceHierarchy.tsx` | Lobby hierarchy integration |
+
+## Data model
+
+| Event | State key | Content |
+|-------|-----------|---------|
+| `im.paarrot.sub_rooms` | `""` (empty) | `{ children: string[] }` — room IDs in display order |
+
+Power levels: treat like other state events; editors need `state_default` or explicit level for `im.paarrot.sub_rooms`.
+
+## Dependencies
+
+- `matrix-js-sdk` state events
+- Parent room must exist and user must have permission to send state
+
+## Integration points
+
+- **Home navigation**: primary consumer of tree flattening
+- **Lobby/forums**: `SpaceHierarchy` reads sub-rooms for display
+- **Create room flow**: auto-updates parent on sub-room create
+- Distinct from **m.space.child** space hierarchy (may coexist)
+
+## Testing
+
+### Manual
+
+1. In a room where you have admin, add sub-room from nav menu.
+2. Verify child appears indented under parent in Home.
+3. Reorder in Room settings → Sub-rooms.
+4. Leave child room; verify unjoined state UI.
+5. Check lobby view shows hierarchy when applicable.
+
+### Automated
+
+- None
+
+## Known issues & gotchas
+
+- Only **joined** sub-rooms appear in home tree; unjoined listed separately
+- Top-level filter: rooms that appear in any parent's `children` are excluded from root list
+- Creating sub-room sends state event on **parent** — failure leaves orphan room
+- `StateEvent.PaarrotSubRooms as any` casts in sendStateEvent — typing debt
+
+## Future work
+
+- Sync with `m.space.child` for federation-friendly hierarchy
+- Drag-and-drop reorder in nav (DnD exists in lobby, not necessarily sub-rooms settings)
+- Depth limits / circular reference guards
+
+## Related docs
+
+- [lobby-forums HANDOFF](../lobby-forums/HANDOFF.md)
+
+## Add your extra things here
+
+- Interface: `PaarrotSubRoomsContent { children: string[] }` in `useRoomSubRooms.ts`
+- Permission check pattern: `permissions.stateEvent(StateEvent.PaarrotSubRooms, userId)`