Fix socket.io connection for reverse proxy deployments

Replace deprecated document.domain + port construction with io()
which auto-connects to the serving host regardless of proxy setup.

templates/base.html

@@ -63,7 +63,7 @@
 
     <script>
       // Connect to the server using socket.io
-      const socket = io.connect(window.location.protocol + "//" + document.domain + ":" + location.port);
+      const socket = io();
     </script>
 
     <!-- Link to the favicon -->