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/vitest.config.ts

@@ -0,0 +1,10 @@
+import { defineConfig } from 'vitest/config';
+
+export default defineConfig({
+  test: {
+    environment: 'jsdom',
+    globals: true,
+    setupFiles: './src/setupTests.ts',
+    include: ['tests/**/*.spec.tsx'],
+  },
+});