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/inbox-and-invites/HANDOFF.md

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+# Inbox & invites — handoff
+
+## Summary
+
+The inbox area covers invites, notification feed, and notification preferences. Sidebar Inbox tab routes to invites list and notifications history.
+
+## User-facing behavior
+
+- View and accept/decline room invites
+- Browse notification events (mentions, etc.)
+- Per-room notification preference overrides
+
+## Architecture
+
+```
+client/inbox/
+  Inbox.tsx — layout
+  Invites.tsx — invite list (allInvitesAtom)
+  Notifications.tsx — notification feed
+ClientRoomsNotificationPreferences.tsx — bulk prefs
+```
+
+## Key files
+
+| Path | Role |
+|------|------|
+| `cinny/src/app/pages/client/inbox/Inbox.tsx` | Inbox layout |
+| `cinny/src/app/pages/client/inbox/Invites.tsx` | Invites UI |
+| `cinny/src/app/pages/client/inbox/Notifications.tsx` | Notifications list |
+| `cinny/src/app/state/room-list/inviteList.ts` | `allInvitesAtom` |
+| `cinny/src/app/pages/client/ClientRoomsNotificationPreferences.tsx` | Room notif prefs |
+| `cinny/src/app/pages/client/sidebar/InboxTab.tsx` | Sidebar entry |
+| `cinny/src/app/hooks/useRoomsNotificationPreferences.ts` | Prefs hook |
+
+## Data model
+
+| Source | Data |
+|--------|------|
+| Sync | `m.room.member` invite membership |
+| Account data | push rules, per-room notification prefs |
+
+## Dependencies
+
+- matrix-js-sdk sync invite events
+- Push rule APIs
+
+## Integration points
+
+- **background-notifications**: same push rules drive system notifications
+- **user-settings**: notification settings tab
+- **routing-and-navigation**: `INBOX_PATH`, `_INVITES_PATH`, `_NOTIFICATIONS_PATH`
+
+## Testing
+
+### Manual
+
+1. Receive invite; appears in inbox.
+2. Accept invite; room joins home list.
+3. Decline invite; removed from list.
+4. Notifications page shows recent highlights.
+
+### Automated
+
+- None
+
+## Known issues & gotchas
+
+- Invite list can desync briefly during sync gap
+- Notification feed scope depends on push rules setup
+
+## Future work
+
+- Mark all invites read
+- Notification grouping by room
+
+## Related docs
+
+- [background-notifications HANDOFF](../background-notifications/HANDOFF.md)
+- [user-settings HANDOFF](../user-settings/HANDOFF.md)
+
+## Add your extra things here
+
+- `getInboxInvitesPath`, `getInboxNotificationsPath` in pathUtils