Update tests and e2e files to support locale-based routing

- Replaced static paths with dynamic locale subpaths (`/[locale]/*`) in imports, URLs, and assertions across tests.
- Updated `next-intl` mocks for improved compatibility with `locale`-aware components.
- Standardized `page.goto` and navigation tests with `/en` as the base locale for consistency.

frontend/tests/app/demos/page.test.tsx

@@ -3,7 +3,7 @@
  */
 
 import { render, screen, within } from '@testing-library/react';
-import DemoTourPage from '@/app/demos/page';
+import DemoTourPage from '@/app/[locale]/demos/page';
 
 // Mock Next.js Link
 jest.mock('next/link', () => ({