Fix WebSocket room subscription format

src/server/web/templates/loading.html

@@ -305,7 +305,9 @@
                 // Subscribe to system room for progress updates
                 ws.send(JSON.stringify({
                     action: 'join',
-                    room: 'system'
+                    data: {
+                        room: 'system'
+                    }
                 }));
             };