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/common/LanguageSelector.tsx

@@ -41,7 +41,7 @@ export const LanguageSelector: React.FC = () => {
     <div className="relative">
       <button
         onClick={() => setIsOpen(!isOpen)}
-        className="flex items-center gap-2 px-3 py-2 text-sm font-medium text-neutral-700 bg-white border border-neutral-300 rounded-lg hover:bg-neutral-50 focus:outline-none focus:ring-2 focus:ring-primary-500"
+        className="flex items-center gap-2 px-3 py-2 text-sm font-medium text-neutral-700 dark:text-neutral-200 bg-white dark:bg-neutral-800 border border-neutral-300 dark:border-neutral-600 rounded-lg hover:bg-neutral-50 dark:hover:bg-neutral-700 focus:outline-none focus:ring-2 focus:ring-primary-500"
       >
         <Globe className="w-4 h-4" />
         <currentLanguage.Flag className="w-5 h-5" />
@@ -49,15 +49,15 @@ export const LanguageSelector: React.FC = () => {
       </button>
 
       {isOpen && (
-        <div className="absolute right-0 mt-2 w-48 bg-white rounded-lg shadow-lg border border-neutral-200 py-1 z-50">
+        <div className="absolute right-0 mt-2 w-48 bg-white dark:bg-neutral-800 rounded-lg shadow-lg border border-neutral-200 dark:border-neutral-700 py-1 z-50">
           {languages.map((language) => (
             <button
               key={language.code}
               onClick={() => handleLanguageChange(language.code)}
-              className={`w-full text-left px-4 py-2 text-sm hover:bg-neutral-50 flex items-center gap-3 ${
+              className={`w-full text-left px-4 py-2 text-sm hover:bg-neutral-50 dark:hover:bg-neutral-700 flex items-center gap-3 ${
                 language.code === i18n.language
-                  ? 'text-primary-600 bg-primary-50'
-                  : 'text-neutral-700'
+                  ? 'text-primary-600 dark:text-primary-400 bg-primary-50 dark:bg-primary-900/30'
+                  : 'text-neutral-700 dark:text-neutral-300'
               }`}
             >
               <language.Flag className="w-5 h-5" />