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/store/index.ts

@@ -32,6 +32,8 @@ interface WasteState {
 
   // chat — keyed by room
   messages: Record<string, ChatMessage[]>
+  // rooms for which we have received history: room → ts of last history message
+  historyCutoff: Record<string, number>
   activeRoom: string
   // user-created rooms, keyed by networkId
   customRooms: Record<string, string[]>
@@ -84,6 +86,7 @@ export const useWaste = create<WasteState>((set, get) => ({
   activeNetworkId: null,
   connectedPeers: [],
   messages: {},
+  historyCutoff: {},
   activeRoom: 'general',
   customRooms: {},
   fileLists: {},
@@ -352,6 +355,24 @@ export const useWaste = create<WasteState>((set, get) => ({
         }
         break
       }
+      case 'history_loaded': {
+        const room = msg.room
+        const incoming = (msg.messages ?? []) as ChatMessage[]
+        if (!room || incoming.length === 0) break
+        set(s => {
+          const existing = s.messages[room] ?? []
+          const existingMids = new Set(existing.map(m => m.mid).filter(Boolean))
+          const fresh = incoming.filter(m => !m.mid || !existingMids.has(m.mid))
+          if (fresh.length === 0) return s
+          const merged = [...fresh, ...existing].sort((a, b) => a.ts - b.ts)
+          const cutoff = fresh[fresh.length - 1]?.ts ?? 0
+          return {
+            messages: { ...s.messages, [room]: merged },
+            historyCutoff: { ...s.historyCutoff, [room]: cutoff },
+          }
+        })
+        break
+      }
     }
   },
 }))