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/src/services/robotsTxtService.js

@@ -0,0 +1,128 @@
+const { db } = require('../database/db');
+
+let cachedRobotsTxt = null;
+let cacheTimestamp = 0;
+const CACHE_TTL_MS = 60 * 1000; // 60 seconds
+
+function parseSetting(raw) {
+  if (raw === null || raw === undefined) return null;
+  if (typeof raw !== 'string') return raw;
+  try {
+    return JSON.parse(raw);
+  } catch {
+    return raw;
+  }
+}
+
+async function getSeoSettings() {
+  const rows = await db('app_settings')
+    .where('setting_type', 'seo')
+    .select('setting_key', 'setting_value');
+
+  const settings = {};
+  for (const row of rows) {
+    settings[row.setting_key] = parseSetting(row.setting_value);
+  }
+  return settings;
+}
+
+const SOCIAL_BOTS = [
+  'Twitterbot',
+  'facebookexternalhit',
+  'LinkedInBot',
+  'Slackbot',
+  'WhatsApp',
+  'TelegramBot',
+  'Discordbot'
+];
+
+async function generateRobotsTxt() {
+  const now = Date.now();
+  if (cachedRobotsTxt && (now - cacheTimestamp) < CACHE_TTL_MS) {
+    return cachedRobotsTxt;
+  }
+
+  const settings = await getSeoSettings();
+  const allowIndexing = settings.seo_allow_indexing === true;
+  const blockAiCrawlers = settings.seo_block_ai_crawlers !== false;
+  const blockSocialBots = settings.seo_block_social_bots === true;
+  const aiAgents = Array.isArray(settings.seo_blocked_ai_agents)
+    ? settings.seo_blocked_ai_agents
+    : [];
+  const customRules = Array.isArray(settings.seo_custom_rules)
+    ? settings.seo_custom_rules
+    : [];
+  const sitemapUrl = settings.seo_sitemap_url || '';
+
+  const lines = [];
+
+  // Always block admin and API paths for all agents
+  lines.push('# Protected paths');
+  lines.push('User-agent: *');
+  lines.push('Disallow: /admin');
+  lines.push('Disallow: /api');
+  lines.push('');
+
+  if (!allowIndexing) {
+    // Block everything for all agents
+    lines.push('# Indexing disabled - block all crawlers');
+    lines.push('User-agent: *');
+    lines.push('Disallow: /');
+    lines.push('');
+  }
+
+  // Block AI crawlers if enabled
+  if (blockAiCrawlers && aiAgents.length > 0) {
+    lines.push('# AI/LLM crawler blocking');
+    for (const agent of aiAgents) {
+      lines.push(`User-agent: ${agent}`);
+      lines.push('Disallow: /');
+      lines.push('');
+    }
+  }
+
+  // Block social bots if enabled
+  if (blockSocialBots) {
+    lines.push('# Social media bot blocking');
+    for (const bot of SOCIAL_BOTS) {
+      lines.push(`User-agent: ${bot}`);
+      lines.push('Disallow: /');
+      lines.push('');
+    }
+  }
+
+  // Custom rules
+  if (customRules.length > 0) {
+    lines.push('# Custom rules');
+    for (const rule of customRules) {
+      if (rule.userAgent && Array.isArray(rule.disallow)) {
+        lines.push(`User-agent: ${rule.userAgent}`);
+        for (const path of rule.disallow) {
+          lines.push(`Disallow: ${path}`);
+        }
+        lines.push('');
+      }
+    }
+  }
+
+  // Sitemap
+  if (sitemapUrl) {
+    lines.push(`Sitemap: ${sitemapUrl}`);
+    lines.push('');
+  }
+
+  const result = lines.join('\n');
+  cachedRobotsTxt = result;
+  cacheTimestamp = now;
+  return result;
+}
+
+function clearRobotsTxtCache() {
+  cachedRobotsTxt = null;
+  cacheTimestamp = 0;
+}
+
+module.exports = {
+  generateRobotsTxt,
+  clearRobotsTxtCache
+};