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/(authenticated)/settings/preferences/page.test.tsx

@@ -4,7 +4,7 @@
  */
 
 import { render, screen } from '@testing-library/react';
-import PreferencesPage from '@/app/(authenticated)/settings/preferences/page';
+import PreferencesPage from '@/app/[locale]/(authenticated)/settings/preferences/page';
 
 describe('PreferencesPage', () => {
   it('renders page title', () => {