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/[locale]/admin/metadata.ts

@@ -0,0 +1,6 @@
+/* istanbul ignore file - Server-only Next.js metadata generation covered by E2E */
+import type { Metadata } from 'next';
+
+export const metadata: Metadata = {
+  title: 'Admin Dashboard',
+};