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/components/admin/AdminAuthWrapper.tsx

@@ -1,12 +1,15 @@
 import React from 'react';
 import { Outlet } from 'react-router-dom';
 import { AdminAuthProvider, PermissionsProvider } from '../../contexts';
+import { AdminDarkModeProvider } from '../../contexts/AdminDarkModeContext';
 
 export const AdminAuthWrapper: React.FC = () => {
   return (
     <AdminAuthProvider>
       <PermissionsProvider>
-        <Outlet />
+        <AdminDarkModeProvider>
+          <Outlet />
+        </AdminDarkModeProvider>
       </PermissionsProvider>
     </AdminAuthProvider>
   );