fix: [Experimental hotfix 1] Fix Signaling issues Toju App

1. Server: WebSocket ping/pong heartbeat (index.ts)
Added a 30-second ping interval that pings all connected clients
Connections without a pong response for 45 seconds are terminated and cleaned up
Extracted removeDeadConnection() to deduplicate the cleanup logic between close events and dead connection reaping
2. Server: Fixed sendServerUsers filter bug (handler.ts:13)
Removed && cu.displayName from the filter — users who joined a server before their identify message was processed were silently invisible to everyone. This was the direct cause of "can't see each other" in session 2.
3. Client: Typing message now includes serverId
Added serverId: this.webrtc.currentServerId to the typing payload
Added a currentServerId getter on WebRTCService

src/app/core/services/webrtc.service.ts

@@ -513,6 +513,11 @@ export class WebRTCService implements OnDestroy {
     this.activeServerId = serverId;
   }
 
+  /** The server ID currently being viewed / active, or `null`. */
+  get currentServerId(): string | null {
+    return this.activeServerId;
+  }
+
   /**
    * Send an identify message to the signaling server.
    *