feat: Add CSS template system with custom gallery styling support

## Changes

### CSS Template System
- Added CSS class hooks to gallery components for custom template targeting
- Gallery sidebar, header, footer, and photo cards can now be styled via CSS templates
- CSS variables on :root allow themes to override colors, effects, and spacing

### Gallery Component CSS Classes Added
- `.gallery-page` - Main gallery container
- `.gallery-header` - Top header bar
- `.gallery-sidebar` - Filter/download sidebar
- `.gallery-sidebar-header`, `.gallery-sidebar-title`, `.gallery-sidebar-close`
- `.gallery-sidebar-content`, `.gallery-sidebar-section`
- `.gallery-sidebar-search-input`, `.gallery-sidebar-search-icon`
- `.gallery-sidebar-backdrop` - Mobile overlay
- `.gallery-btn`, `.gallery-btn-download` - Sidebar buttons
- `.gallery-footer` - Footer section
- `.photo-card`, `.photo-grid` - Photo display elements

### CSS Templates (Database)
- Elegant Dark (id=1): Dark navy theme with light text and red accents
- Liquid Glass Light (id=2): iOS 26 frosted glass effect with gradient background

### Bug Fixes
- Fixed CSS variables not inheriting (moved from .gallery-page to :root)
- Fixed sidebar position breaking layout (removed position: relative override)
- Fixed Elegant Dark sidebar text visibility (white on white issue)

### Other Changes
- Settings page refactoring and cleanup
- i18n locale updates for new gallery features
- Vite proxy port configuration fix
- Admin auth route improvements
- CSS templates service updates

backend/server.js

@@ -25,6 +25,7 @@ const { startBackupService } = require('./src/services/backupService');
 const { startScheduledBackups } = require('./src/services/databaseBackup');
 const { maintenanceMiddleware } = require('./src/middleware/maintenance');
 const { sessionTimeoutMiddleware } = require('./src/middleware/sessionTimeout');
+const { errorHandler, notFoundHandler } = require('./src/middleware/errorHandler');
 const { createRateLimiter, createAuthRateLimiter } = require('./src/services/rateLimitService');
 const { getPublicSitePayload } = require('./src/services/publicSiteService');
 const cookieParser = require('cookie-parser');
@@ -471,20 +472,11 @@ try {
   logger.warn('Failed to enable frontend static serving', { error: e.message });
 }
 
-// Error handling middleware
-app.use((err, req, res, next) => {
-  console.error('EXPRESS ERROR HANDLER:', err);
-  console.error('Error stack:', err.stack);
-  console.error('Request URL:', req.url);
-  console.error('Request method:', req.method);
-  logger.error('Express error handler:', {
-    message: err.message,
-    stack: err.stack,
-    url: req.url,
-    method: req.method
-  });
-  res.status(500).json({ error: 'Something went wrong!', details: err.message });
-});
+// 404 handler for undefined API routes
+app.use('/api', notFoundHandler);
+
+// Global error handler (must be last)
+app.use(errorHandler);
 
 // Initialize services
 async function startServer() {