feat: add admin dark mode and SEO/robots.txt settings

Admin Dark Mode:
- Add AdminDarkModeContext with light/dark/system preference
- Update all admin components with Tailwind dark: classes
- Add dark mode toggle in admin header
- Persist preference in localStorage

SEO Settings:
- Add robots.txt configuration in Settings > SEO tab
- Block AI crawlers (GPTBot, ChatGPT-User, etc.) with toggle
- Custom robots.txt rules management
- Add RobotsMetaTags component for gallery pages
- Backend service for dynamic robots.txt generation
- Database migration for SEO settings storage

UI/UX Improvements:
- Consistent dark mode styling across all admin pages
- Update gallery components with themed CSS classes
- Fix input, card, and button styling for dark mode

frontend/src/services/settings.service.ts

@@ -240,6 +240,8 @@ export const settingsService = {
       endpoint = '/admin/settings/analytics';
     } else if (firstKey?.startsWith('branding_')) {
       endpoint = '/admin/settings/branding';
+    } else if (firstKey?.startsWith('seo_')) {
+      endpoint = '/admin/settings/seo';
     }
     
     await api.put(endpoint, settings);