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

backend/migrations/core/068_add_seo_robots_settings.js

@@ -0,0 +1,54 @@
+const DEFAULT_AI_AGENTS = [
+  'GPTBot',
+  'ChatGPT-User',
+  'Google-Extended',
+  'Claude-Web',
+  'Anthropic-AI',
+  'CCBot',
+  'Bytespider',
+  'FacebookBot',
+  'Omgilibot',
+  'Diffbot',
+  'PetalBot',
+  'Amazonbot',
+  'PerplexityBot',
+  'YouBot',
+  'Applebot-Extended'
+];
+
+exports.up = async function(knex) {
+  const defaults = [
+    { setting_key: 'seo_allow_indexing', setting_value: JSON.stringify(false), setting_type: 'seo' },
+    { setting_key: 'seo_block_ai_crawlers', setting_value: JSON.stringify(true), setting_type: 'seo' },
+    { setting_key: 'seo_block_social_bots', setting_value: JSON.stringify(false), setting_type: 'seo' },
+    { setting_key: 'seo_blocked_ai_agents', setting_value: JSON.stringify(DEFAULT_AI_AGENTS), setting_type: 'seo' },
+    { setting_key: 'seo_custom_rules', setting_value: JSON.stringify([]), setting_type: 'seo' },
+    { setting_key: 'seo_meta_noindex', setting_value: JSON.stringify(true), setting_type: 'seo' },
+    { setting_key: 'seo_meta_nofollow', setting_value: JSON.stringify(false), setting_type: 'seo' },
+    { setting_key: 'seo_meta_noai', setting_value: JSON.stringify(true), setting_type: 'seo' },
+    { setting_key: 'seo_sitemap_url', setting_value: JSON.stringify(''), setting_type: 'seo' }
+  ];
+
+  for (const setting of defaults) {
+    const exists = await knex('app_settings').where('setting_key', setting.setting_key).first();
+    if (!exists) {
+      await knex('app_settings').insert({ ...setting, updated_at: knex.fn.now() });
+    }
+  }
+};
+
+exports.down = async function(knex) {
+  await knex('app_settings')
+    .whereIn('setting_key', [
+      'seo_allow_indexing',
+      'seo_block_ai_crawlers',
+      'seo_block_social_bots',
+      'seo_blocked_ai_agents',
+      'seo_custom_rules',
+      'seo_meta_noindex',
+      'seo_meta_nofollow',
+      'seo_meta_noai',
+      'seo_sitemap_url'
+    ])
+    .del();
+};