refactor(admin): move event details page to admin section

- Moved EventDetailsPage from /events/:slug/details to /admin/events/:slug/details
- Added admin authentication check with redirect to login/home
- Updated all navigation links across the app:
  - EventsPage: "View details (admin)" button
  - EventChatPage: "View QR Code (admin)" link
  - EventCard: handleViewDetails navigation
- Fixed relative imports after moving to admin folder

This page contains admin-only features (QR codes, participants list,
matching config, scheduler config, matching runs) and should only be
accessible to administrators.

frontend/src/pages/EventChatPage.jsx

@@ -380,10 +380,10 @@ const EventChatPage = () => {
                 </Link>
                 {import.meta.env.DEV && (
                   <Link
-                    to={`/events/${slug}/details`}
+                    to={`/admin/events/${slug}/details`}
                     className="px-6 py-2 bg-primary-600 text-white rounded-lg hover:bg-primary-700 transition-colors"
                   >
-                    View QR Code (dev)
+                    View QR Code (admin)
                   </Link>
                 )}
               </div>

frontend/src/pages/EventsPage.jsx

@@ -180,10 +180,10 @@ const EventsPage = () => {
           {/* Development mode: Show details link */}
           {import.meta.env.DEV && (
             <button
-              onClick={() => navigate(`/events/${event.slug}/details`)}
+              onClick={() => navigate(`/admin/events/${event.slug}/details`)}
               className="w-full px-4 py-2 border border-gray-300 text-gray-700 rounded-md hover:bg-gray-50 transition-colors text-sm"
             >
-              View details (dev)
+              View details (admin)
             </button>
           )}
         </div>

frontend/src/pages/admin/EventDetailsPage.jsx

@@ -1,23 +1,42 @@
 import { useState, useEffect } from 'react';
-import { useParams, Link } from 'react-router-dom';
+import { useParams, Link, useNavigate } from 'react-router-dom';
 import { Calendar, MapPin } from 'lucide-react';
-import Layout from '../components/layout/Layout';
-import { eventsAPI } from '../services/api';
-import QRCodeSection from '../components/events/QRCodeSection';
-import ParticipantsSection from '../components/events/ParticipantsSection';
-import MatchingConfigSection from '../components/events/MatchingConfigSection';
-import ScheduleConfigSection from '../components/events/ScheduleConfigSection';
-import MatchingRunsSection from '../components/events/MatchingRunsSection';
+import Layout from '../../components/layout/Layout';
+import { eventsAPI } from '../../services/api';
+import QRCodeSection from '../../components/events/QRCodeSection';
+import ParticipantsSection from '../../components/events/ParticipantsSection';
+import MatchingConfigSection from '../../components/events/MatchingConfigSection';
+import ScheduleConfigSection from '../../components/events/ScheduleConfigSection';
+import MatchingRunsSection from '../../components/events/MatchingRunsSection';
 
 export default function EventDetailsPage() {
   const { slug } = useParams();
+  const navigate = useNavigate();
+  const [user, setUser] = useState(null);
   const [eventDetails, setEventDetails] = useState(null);
   const [loading, setLoading] = useState(true);
   const [error, setError] = useState('');
 
+  // Check if user is admin
   useEffect(() => {
-    fetchEventDetails();
-  }, [slug]);
+    const userData = localStorage.getItem('user');
+    if (userData) {
+      const parsedUser = JSON.parse(userData);
+      setUser(parsedUser);
+
+      if (!parsedUser.isAdmin) {
+        navigate('/');
+      }
+    } else {
+      navigate('/login');
+    }
+  }, [navigate]);
+
+  useEffect(() => {
+    if (user?.isAdmin) {
+      fetchEventDetails();
+    }
+  }, [slug, user]);
 
   const fetchEventDetails = async () => {
     try {