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/App.tsx

@@ -1,4 +1,4 @@
-import { useEffect } from 'react';
+import { useEffect, useState } from 'react';
 import { BrowserRouter as Router, Routes, Route, Navigate } from 'react-router-dom';
 import { QueryClient, QueryClientProvider } from '@tanstack/react-query';
 import { ToastContainer } from 'react-toastify';
@@ -29,7 +29,7 @@ import {
 } from './pages/admin';
 import { AcceptInvitePage } from './pages/public/AcceptInvitePage';
 import { AdminLayout, AdminAuthWrapper } from './components/admin';
-import { PageErrorBoundary, OfflineIndicator, SkipLink, DynamicFavicon } from './components/common';
+import { PageErrorBoundary, OfflineIndicator, SkipLink, DynamicFavicon, RobotsMetaTags } from './components/common';
 import { MaintenanceWrapper } from './components/MaintenanceWrapper';
 import { GlobalThemeProvider } from './components/GlobalThemeProvider';
 import { getApiBaseUrl } from './utils/url';
@@ -45,6 +45,17 @@ const queryClient = new QueryClient({
 });
 
 function App() {
+  // Track dark mode for toast theming
+  const [toastTheme, setToastTheme] = useState<'light' | 'dark'>('light');
+
+  useEffect(() => {
+    const observer = new MutationObserver(() => {
+      setToastTheme(document.documentElement.classList.contains('dark') ? 'dark' : 'light');
+    });
+    observer.observe(document.documentElement, { attributes: true, attributeFilter: ['class'] });
+    return () => observer.disconnect();
+  }, []);
+
   // Initialize Umami Analytics based on settings
   useEffect(() => {
     const initializeAnalytics = async () => {
@@ -103,6 +114,7 @@ function App() {
           <ThemeProvider>
             <GlobalThemeProvider>
               <DynamicFavicon />
+              <RobotsMetaTags />
               <Router>
                 <MaintenanceWrapper>
                   <SkipLink />
@@ -165,7 +177,7 @@ function App() {
               pauseOnFocusLoss
               draggable
               pauseOnHover
-              theme="light"
+              theme={toastTheme}
             />
             </GlobalThemeProvider>
           </ThemeProvider>