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/services/imageProcessor.js

@@ -3,15 +3,17 @@ const path = require('path');
 const fs = require('fs').promises;
 
 const THUMBNAIL_WIDTH = 300;
-const THUMBNAIL_PATH = path.join(__dirname, '../../../storage/thumbnails');
+const getStoragePath = () => process.env.STORAGE_PATH || path.join(__dirname, '../../../storage');
+const getThumbnailPath = () => path.join(getStoragePath(), 'thumbnails');
 
 async function generateThumbnail(imagePath) {
   const filename = path.basename(imagePath);
   const thumbnailFilename = `thumb_${filename}`;
-  const thumbnailPath = path.join(THUMBNAIL_PATH, thumbnailFilename);
+  const thumbnailDir = getThumbnailPath();
+  const thumbnailPath = path.join(thumbnailDir, thumbnailFilename);
   
   // Ensure thumbnail directory exists
-  await fs.mkdir(THUMBNAIL_PATH, { recursive: true });
+  await fs.mkdir(thumbnailDir, { recursive: true });
   
   // Generate thumbnail
   await sharp(imagePath)
@@ -22,7 +24,7 @@ async function generateThumbnail(imagePath) {
     .jpeg({ quality: 80 })
     .toFile(thumbnailPath);
   
-  return path.relative(path.join(__dirname, '../../../storage'), thumbnailPath);
+  return path.relative(getStoragePath(), thumbnailPath);
 }
 
 module.exports = { generateThumbnail };