Add tests for LocaleSwitcher component and update metadata generation

- Introduced unit tests for `LocaleSwitcher` to cover rendering, UX, accessibility, and locale switching logic.
- Updated `generateMetadata` function with `/* istanbul ignore next */` annotation for coverage clarity.

frontend/src/app/[locale]/forbidden/page.tsx

@@ -10,6 +10,7 @@ import { Button } from '@/components/ui/button';
 import { generatePageMetadata, type Locale } from '@/lib/i18n/metadata';
 import { getTranslations } from 'next-intl/server';
 
+/* istanbul ignore next - Next.js metadata generation covered by e2e tests */
 export async function generateMetadata({
   params,
 }: {