feat: scaffold frontend (Vite + React + Vitest)

- Añade scaffold de frontend con Vite y React
- Configura Vitest y tests básicos (App, Navbar)
- Añade QueryClient y hooks/plantillas iniciales

frontend/src/main.tsx

@@ -0,0 +1,32 @@
+import React from 'react';
+import { createRoot } from 'react-dom/client';
+import { QueryClientProvider } from '@tanstack/react-query';
+import { queryClient } from './lib/queryClient';
+import App from './App';
+import './styles.css';
+
+const rootEl = document.getElementById('root');
+
+if (rootEl) {
+  createRoot(rootEl).render(
+    <React.StrictMode>
+      <QueryClientProvider client={queryClient}>
+        <App />
+      </QueryClientProvider>
+    </React.StrictMode>
+  );
+}
+import React from 'react';
+import ReactDOM from 'react-dom/client';
+import { QueryClientProvider } from '@tanstack/react-query';
+import { queryClient } from './lib/queryClient';
+import App from './App';
+import './index.css';
+
+ReactDOM.createRoot(document.getElementById('root')!).render(
+  <React.StrictMode>
+    <QueryClientProvider client={queryClient}>
+      <App />
+    </QueryClientProvider>
+  </React.StrictMode>
+);