Refine unread message count logic to reset only for the selected server.

frontend/src/app/pages/altitude/chat/service/chat.service.ts

@@ -86,6 +86,13 @@ export class ChatService implements OnDestroy {
 
   public selectServer(server: string) {
     this._selectedServer.set(server);
+    this._servers.update((old) =>
+      old.map((chatServer) =>
+        chatServer.server === server
+          ? {...chatServer, unreadMessages: 0}
+          : chatServer
+      )
+    );
   }
 
 // single, deliberate escape hatch from the broken DOM/polyfill type overlap —