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/package.json

@@ -11,14 +11,21 @@
   },
   "dependencies": {
     "@tanstack/react-query": "^5.0.0",
+    "@tiptap/extension-link": "^2.25.0",
+    "@tiptap/react": "^2.25.0",
+    "@tiptap/starter-kit": "^2.25.0",
     "axios": "^1.3.2",
     "clsx": "^2.0.0",
     "date-fns": "^2.29.3",
+    "i18next": "^25.3.1",
+    "i18next-browser-languagedetector": "^8.2.0",
+    "i18next-http-backend": "^3.0.2",
     "js-cookie": "^3.0.5",
     "lucide-react": "^0.292.0",
     "react": "^18.3.1",
     "react-countdown": "^2.3.5",
     "react-dom": "^18.3.1",
+    "react-i18next": "^15.6.0",
     "react-image-gallery": "^1.2.11",
     "react-intersection-observer": "^9.4.3",
     "react-router-dom": "^6.8.0",