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/message-search/HANDOFF.md

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+# Message search — handoff
+
+## Summary
+
+In-room and cross-room message search UI: query input, filters, result grouping, and navigation to found events. Uses Matrix `/search` or client-side APIs via `useMessageSearch`.
+
+## User-facing behavior
+
+- Search modal or page with text query
+- Filters: room, sender, date (see SearchFilters)
+- Results grouped by room/period
+- Click result → jump to event in timeline
+
+## Architecture
+
+```
+MessageSearch.tsx
+  → useMessageSearch.ts
+  → SearchInput, SearchFilters, SearchResultGroup
+Router may mount search under client routes
+```
+
+## Key files
+
+| Path | Role |
+|------|------|
+| `cinny/src/app/features/message-search/MessageSearch.tsx` | Main UI |
+| `cinny/src/app/features/message-search/useMessageSearch.ts` | Search logic |
+| `cinny/src/app/features/message-search/SearchInput.tsx` | Query field |
+| `cinny/src/app/features/message-search/SearchFilters.tsx` | Filters |
+| `cinny/src/app/features/message-search/SearchResultGroup.tsx` | Result list |
+
+## Data model
+
+Ephemeral search state in component hooks. No persistence.
+
+## Dependencies
+
+- `matrix-js-sdk` search API
+- Homeserver must support message search (server-dependent)
+
+## Integration points
+
+- **routing-and-navigation**: search routes in home/space
+- **room-timeline**: navigation to event id
+- **global-search**: separate modal feature — see that handoff
+
+## Testing
+
+### Manual
+
+1. Search known keyword; results appear.
+2. Apply room filter; results scoped.
+3. Click result; timeline scrolls to event.
+4. Test on homeserver without search — expect graceful error.
+
+### Automated
+
+- None
+
+## Known issues & gotchas
+
+- Server-side search availability varies by deployment
+- Encrypted room search limited by server/indexing policy
+- Pagination in `useMessageSearch` — check for duplicate results
+
+## Future work
+
+- Client-side fallback search for small rooms
+- Highlight match in timeline
+
+## Related docs
+
+- [global-search HANDOFF](../global-search/HANDOFF.md)
+- [room-timeline HANDOFF](../room-timeline/HANDOFF.md)
+
+## Add your extra things here
+
+- Exported from `features/message-search/index.ts`