feat(websocket): add ping/pong mechanism to maintain connection

- Add proxy timeouts in nginx.conf to handle long connections
- Implement ping/pong handling in server.js WebSocket listener
- Refactor App.js to class component with lifecycle methods and ping interval for WebSocket keep-alive

nginx.conf

@@ -21,4 +21,6 @@
         proxy_set_header X-Real-IP $remote_addr;
         proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
         proxy_set_header X-Forwarded-Proto $scheme;
+        proxy_read_timeout 86400;
+        proxy_send_timeout 86400;
     }