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 <[email protected]>

frontend/src/main.tsx

@@ -1,6 +1,7 @@
 import { StrictMode } from 'react'
 import { createRoot } from 'react-dom/client'
 import './index.css'
+import './i18n/config'
 import App from './App.tsx'
 
 createRoot(document.getElementById('root')!).render(