Fix brand theme application and add comprehensive translations

- Fixed theme not being reflected on gallery and admin login pages
- Created GlobalThemeProvider to apply themes globally
- Updated gallery and admin login pages to use dynamic CSS variables
- Added complete translations for all admin sections in English and German:
  - Notifications management
  - Event view and creation
  - Photo upload functionality
  - Category management
  - Archive page view
  - Analytics dashboard
  - Branding and theme settings
  - System settings
  - CMS page management
  - Email configuration
- Fixed admin photo management display issues
- Fixed photo upload category assignment
- Added password reset functionality for galleries
- Improved error handling and user feedback

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

frontend/src/components/admin/PhotoUpload.tsx

@@ -61,9 +61,7 @@ export const PhotoUpload: React.FC<PhotoUploadProps> = ({ eventId, onUploadCompl
 
     try {
       const response = await api.post(`/api/admin/events/${eventId}/upload`, formData, {
-        headers: {
-          'Content-Type': 'multipart/form-data',
-        },
+        // Don't set Content-Type header - axios will set it with the boundary
         onUploadProgress: (progressEvent) => {
           if (progressEvent.total) {
             const progress = Math.round((progressEvent.loaded * 100) / progressEvent.total);