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/tests/App.spec.tsx

@@ -0,0 +1,19 @@
+import React from 'react';
+import { render, screen } from '@testing-library/react';
+import App from '../src/App';
+
+describe('App', () => {
+  it('renderiza el título Quasar', () => {
+    render(<App />);
+    expect(screen.getByText('Quasar')).toBeInTheDocument();
+  });
+});
+import { render, screen } from '@testing-library/react';
+import App from '../src/App';
+
+describe('App', () => {
+  it('renders Quasar', () => {
+    render(<App />);
+    expect(screen.getByText(/Quasar/i)).toBeInTheDocument();
+  });
+});