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/features/chat/chat-messages/chat-messages.component.ts

@@ -107,7 +107,7 @@ export class ChatMessagesComponent {
 
   handleTypingStarted(): void {
     try {
-      this.webrtc.sendRawMessage({ type: 'typing' });
+      this.webrtc.sendRawMessage({ type: 'typing', serverId: this.webrtc.currentServerId });
     } catch {
       /* ignore */
     }