Fix language setting not being saved to database on admin settings page

- Added default_language field to general settings state in SettingsPage
- Replaced LanguageSelector component with simple select dropdown on settings page
- Fixed public settings endpoint to read general_default_language from database
- Language setting now properly saved when clicking Save Settings button
- Setting is correctly used by gallery login page and legal pages

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

frontend/src/components/common/index.ts

@@ -12,4 +12,7 @@ export {
   SkeletonList 
 } from './Skeleton';
 export { OfflineIndicator, useOnlineStatus } from './OfflineIndicator';
-export { SkipLink } from './SkipLink';
+export { SkipLink } from './SkipLink';
+export { DynamicFavicon } from './DynamicFavicon';
+export { LanguageSelector } from './LanguageSelector';
+export { AuthenticatedImage } from './AuthenticatedImage';