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>

backend/src/routes/admin.js

@@ -8,6 +8,8 @@ const emailRoutes = require('./adminEmail');
 const settingsRoutes = require('./adminSettings');
 const eventsRoutes = require('./adminEvents');
 const photosRoutes = require('./adminPhotos');
+const categoriesRoutes = require('./adminCategories');
+const cmsRoutes = require('./adminCMS');
 
 // Mount sub-routers
 router.use('/dashboard', dashboardRoutes);
@@ -16,5 +18,7 @@ router.use('/email', emailRoutes);
 router.use('/settings', settingsRoutes);
 router.use('/events', eventsRoutes);
 router.use('/events', photosRoutes);
+router.use('/categories', categoriesRoutes);
+router.use('/cms', cmsRoutes);
 
 module.exports = router;