Add React frontend and clean up legacy HTTP backend

frontend/src/main.tsx

@@ -0,0 +1,14 @@
+import React from 'react';
+import ReactDOM from 'react-dom/client';
+import App from './App';
+import './index.css';
+import { setupSocketListeners, socket } from './lib/socket';
+
+setupSocketListeners();
+socket.connect();
+
+ReactDOM.createRoot(document.getElementById('root')!).render(
+  <React.StrictMode>
+    <App />
+  </React.StrictMode>
+);