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>

backend/package.json

@@ -11,7 +11,9 @@
     "lint": "eslint src/"
   },
   "dependencies": {
+    "adm-zip": "^0.5.16",
     "archiver": "^5.3.1",
+    "axios": "^1.10.0",
     "bcrypt": "^5.1.0",
     "chokidar": "^3.5.3",
     "cors": "^2.8.5",
@@ -19,6 +21,7 @@
     "express": "^4.18.2",
     "express-rate-limit": "^6.7.0",
     "express-validator": "^7.0.1",
+    "form-data": "^4.0.3",
     "helmet": "^7.0.0",
     "i18next": "^25.3.1",
     "i18next-browser-languagedetector": "^8.2.0",