e2e-test: preserve next-app styles in React 17 bootstrap

Keep the generated app stylesheet import when rewriting the new frontend app entrypoint for React 17 so the E2E app boots with the same UI setup.

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>
Made-with: Cursor

packages/e2e-test/src/commands/runCommand.ts

@@ -436,6 +436,7 @@ async function switchToReact17(appDir: string) {
     `import '@backstage/cli/asset-types';
 import ReactDOM from 'react-dom';
 import App from './App';
+import '@backstage/ui/css/styles.css';
 
 ReactDOM.render(App.createRoot(), document.getElementById('root'));
 `,