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/components/gallery/PhotoLightbox.tsx

@@ -2,6 +2,7 @@ import React, { useState, useEffect } from 'react';
 import { X, ChevronLeft, ChevronRight, Download, ZoomIn, ZoomOut } from 'lucide-react';
 import type { Photo } from '../../types';
 import { useDownloadPhoto } from '../../hooks/useGallery';
+import { AuthenticatedImage } from '../common';
 
 interface PhotoLightboxProps {
   photos: Photo[];
@@ -241,7 +242,7 @@ export const PhotoLightbox: React.FC<PhotoLightboxProps> = ({
         onTouchEnd={handleTouchEnd}
         style={{ cursor: zoom > 1 ? (isDragging ? 'grabbing' : 'grab') : 'default' }}
       >
-        <img
+        <AuthenticatedImage
           src={currentPhoto.url}
           alt={currentPhoto.filename}
           className="max-w-full max-h-full object-contain select-none"
@@ -250,6 +251,7 @@ export const PhotoLightbox: React.FC<PhotoLightboxProps> = ({
             transition: isDragging ? 'none' : 'transform 0.2s',
           }}
           draggable={false}
+          useWatermark={true}
         />
       </div>