feat: render history_loaded in web UI with earlier messages divider

- 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>

web/src/types.ts

@@ -82,6 +82,9 @@ export type IpcMsgType =
   | 'shares_list'
   | 'peer_status'
   | 'error'
+  | 'history_loaded'
+  | 'room_created'
+  | 'create_room'
 
 export interface IpcMessage {
   type: IpcMsgType
@@ -124,4 +127,6 @@ export interface IpcMessage {
   conn_state?: PeerConnState
   candidate_type?: CandidateType
   remote_address?: string
+  // history_loaded
+  messages?: ChatMessage[]
 }