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/sitemap.ts

@@ -1,3 +1,4 @@
+/* istanbul ignore file - Framework-only metadata route covered by E2E */
 import { MetadataRoute } from 'next';
 import { routing } from '@/lib/i18n/routing';
 
@@ -5,7 +6,6 @@ import { routing } from '@/lib/i18n/routing';
  * Generate multilingual sitemap
  * Includes all public routes for each supported locale
  */
-/* istanbul ignore next - Next.js metadata route covered by e2e tests */
 export default function sitemap(): MetadataRoute.Sitemap {
   const baseUrl = process.env.NEXT_PUBLIC_SITE_URL || 'http://localhost:3000';