chore: sync infra docs and coverage

frontend/src/App.test.js

@@ -1,5 +1,5 @@
 import React from 'react';
-import ReactDOMServer from 'react-dom/server';
+import { render, screen } from '@testing-library/react';
 import App from './App';
 
 jest.mock('axios', () => ({
@@ -49,7 +49,7 @@ jest.mock('react-hot-toast', () => ({
 
 describe('App', () => {
   it('renders without crashing', () => {
-    const markup = ReactDOMServer.renderToStaticMarkup(<App />);
-    expect(markup).toContain('data-testid="toaster"');
+    render(<App />);
+    expect(screen.getByTestId('toaster')).toBeInTheDocument();
   });
 });