Refactor metadata handling for improved maintainability and localization support

- Extracted server-only metadata generation logic into separate files, reducing inline logic in page components.
- Added `/* istanbul ignore file */` annotations for E2E-covered framework-level metadata.
- Standardized `generateMetadata` export patterns across auth, admin, and error pages for consistency.
- Enhanced maintainability and readability by centralizing metadata definitions for each route.

frontend/src/app/page.tsx

@@ -1,10 +1,10 @@
+/* istanbul ignore file - Framework-only root redirect covered by E2E */
 /**
  * Root page - redirects to default locale
  */
 
 import { redirect } from 'next/navigation';
 
-/* istanbul ignore next - Next.js server-side redirect covered by e2e tests */
 export default function RootPage() {
   // Redirect to default locale (en)
   redirect('/en');