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/admin/layout.test.tsx

@@ -5,7 +5,7 @@
 
 import { render, screen } from '@testing-library/react';
 import { QueryClient, QueryClientProvider } from '@tanstack/react-query';
-import AdminLayout from '@/app/admin/layout';
+import AdminLayout from '@/app/[locale]/admin/layout';
 import { useAuth } from '@/lib/auth/AuthContext';
 
 // Mock dependencies