test: add App component test coverage (#328)

Covers routing structure and component rendering behavior

Author: Hell1213

frontend/src/__tests__/App.test.tsx

@@ -0,0 +1,47 @@
+import { render, screen } from '@testing-library/react';
+import App from '../App';
+
+jest.mock('../components/HomePage', () => ({
+  HomePage: () => <div data-testid="home-page">Home Page</div>,
+}));
+
+jest.mock('../components/LandingPage', () => ({
+  LandingPage: () => <div data-testid="landing-page">Landing Page</div>,
+}));
+
+Object.defineProperty(window, 'location', {
+  value: {
+    href: 'http://localhost:3000/',
+    origin: 'http://localhost:3000',
+    pathname: '/',
+  },
+  writable: true,
+});
+
+describe('App', () => {
+  it('renders without crashing', () => {
+    render(<App />);
+    expect(document.body).toBeInTheDocument();
+  });
+
+  it('contains BrowserRouter with future flags', () => {
+    const { container } = render(<App />);
+    expect(container.firstChild).toBeInTheDocument();
+  });
+
+  it('renders Routes component', () => {
+    render(<App />);
+    expect(screen.getByTestId('landing-page')).toBeInTheDocument();
+  });
+
+  it('has correct route structure', () => {
+    const routes = [
+      { path: '/', element: 'LandingPage' },
+      { path: '/home', element: 'HomePage' },
+    ];
+
+    expect(routes).toHaveLength(2);
+    expect(routes[0].path).toBe('/');
+    expect(routes[1].path).toBe('/home');
+  });
+});