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/AdminLayout.tsx

@@ -17,7 +17,7 @@ export const AdminLayout: React.FC = () => {
 
   if (isLoading) {
     return (
-      <div className="min-h-screen bg-neutral-50 flex items-center justify-center">
+      <div className="min-h-screen bg-neutral-50 dark:bg-neutral-950 flex items-center justify-center">
         <div className="text-center">
           <div className="w-16 h-16 border-4 border-primary-600 border-t-transparent rounded-full animate-spin mx-auto mb-4"></div>
           <p className="text-neutral-600">Loading...</p>
@@ -31,7 +31,7 @@ export const AdminLayout: React.FC = () => {
   }
 
   return (
-    <div className="h-screen bg-neutral-50 flex overflow-hidden">
+    <div className="h-screen bg-neutral-50 dark:bg-neutral-950 flex overflow-hidden">
       {/* Mandatory Password Change Modal */}
       {mustChangePassword && <MandatoryPasswordChangeModal />}