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/join-before-navigate/HANDOFF.md

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+# Join before navigate — handoff
+
+## Summary
+
+When a user opens a room link for a room they have not joined, this preview page shows room info (name, topic, member count) and offers join before entering the timeline.
+
+## User-facing behavior
+
+- matrix.to / internal join links land here if not member
+- Room card preview with topic
+- Join button → membership then navigate to room/space
+- Back navigation on mobile
+
+## Architecture
+
+```
+JoinBeforeNavigate.tsx
+  → RoomSummaryLoader for preview data
+  → useRoomNavigate after join
+Route: _JOIN_PATH in Router.tsx
+```
+
+## Key files
+
+| Path | Role |
+|------|------|
+| `cinny/src/app/features/join-before-navigate/JoinBeforeNavigate.tsx` | Preview + join UI |
+| `cinny/src/app/components/RoomSummaryLoader.tsx` | Fetch room preview |
+| `cinny/src/app/hooks/useRoomNavigate.ts` | Post-join navigation |
+| `cinny/src/app/state/joiningProgress.ts` | Join in progress UX |
+
+## Data model
+
+Preview from `/summary` API or peek room state. No local persistence.
+
+## Dependencies
+
+- matrix-js-sdk `joinRoom`
+- `via` servers param for federated joins
+
+## Integration points
+
+- **routing-and-navigation**: join route with `roomIdOrAlias`, `viaServers`
+- **explore**: similar preview patterns
+- **electron-shell**: `paarrot://` deep links may target join flow
+
+## Testing
+
+### Manual
+
+1. Open link to unjoined public room.
+2. Preview shows name/topic.
+3. Join succeeds; lands in timeline.
+4. Space link navigates to space not room.
+
+### Automated
+
+- None
+
+## Known issues & gotchas
+
+- Private rooms without peek show limited preview
+- `viaServers` required for some federated aliases
+- `isSpace` check routes to space navigation after join
+
+## Future work
+
+- Knock rooms support
+- Invite-only room messaging
+
+## Related docs
+
+- [routing-and-navigation HANDOFF](../routing-and-navigation/HANDOFF.md)
+- [electron-shell HANDOFF](../electron-shell/HANDOFF.md)
+
+## Add your extra things here
+
+- Props: `roomIdOrAlias`, optional `eventId`, `viaServers[]`