- Remove clearRoomMessages on room switch: IDB cache now persists across
visits, enabling instant re-entry without API round-trip
- Increase initial API limit from 50 → 200: more messages cached upfront
- Add loadOlderMessagesFromIdb: uses 'by_room_seq' compound index to
serve scroll-back history from IDB without API call
- loadMore now tries IDB first before falling back to API