- store: handle history_loaded event — prepend gossipped messages,
dedup by mid, sort by ts, record cutoff timestamp per room
- MessagePane: show "earlier messages" divider between history and
live messages based on the cutoff timestamp
- types: add history_loaded, room_created, create_room to IpcMsgType;
add messages field to IpcMessage
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>