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/admin/EventDetailsPage.jsx

@@ -0,0 +1,173 @@
+import { useState, useEffect } from 'react';
+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';
+
+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(() => {
+    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 {
+      setLoading(true);
+      const response = await eventsAPI.getDetails(slug);
+      setEventDetails(response.data);
+    } catch (err) {
+      console.error('Error loading event details:', err);
+      setError(err.message || 'Failed to load event details');
+    } finally {
+      setLoading(false);
+    }
+  };
+
+  const formatDate = (dateString) => {
+    return new Date(dateString).toLocaleDateString('en-US', {
+      year: 'numeric',
+      month: 'long',
+      day: 'numeric',
+    });
+  };
+
+  if (loading) {
+    return (
+      <Layout>
+        <div className="flex items-center justify-center min-h-screen">
+          <div className="text-center">
+            <div className="animate-spin rounded-full h-12 w-12 border-b-2 border-primary-600 mx-auto"></div>
+            <p className="mt-4 text-gray-600">Loading event details...</p>
+          </div>
+        </div>
+      </Layout>
+    );
+  }
+
+  if (error) {
+    return (
+      <Layout>
+        <div className="max-w-4xl mx-auto p-6">
+          <div className="bg-red-50 border border-red-200 rounded-lg p-4">
+            <p className="text-red-800">{error}</p>
+            <Link to="/events" className="text-red-600 hover:underline mt-2 inline-block">
+              Back to Events
+            </Link>
+          </div>
+        </div>
+      </Layout>
+    );
+  }
+
+  if (!eventDetails) {
+    return null;
+  }
+
+  const { event, checkin, participants, stats } = eventDetails;
+
+  return (
+    <Layout>
+      <div className="max-w-6xl mx-auto p-6">
+        {/* Header */}
+        <div className="mb-6">
+          <Link to="/events" className="text-primary-600 hover:underline mb-2 inline-block">
+            &larr; Back to Events
+          </Link>
+          <h1 className="text-3xl font-bold text-gray-900">{event.name}</h1>
+          <div className="flex items-center gap-4 mt-2 text-gray-600">
+            <div className="flex items-center gap-1">
+              <MapPin size={16} />
+              <span>{event.location}</span>
+            </div>
+            <div className="flex items-center gap-1">
+              <Calendar size={16} />
+              <span>{formatDate(event.startDate)} - {formatDate(event.endDate)}</span>
+            </div>
+          </div>
+        </div>
+
+        {/* QR Code & Participants Grid */}
+        <div className="grid md:grid-cols-2 gap-6">
+          <QRCodeSection checkin={checkin} formatDate={formatDate} />
+          <ParticipantsSection participants={participants} totalCount={stats.totalParticipants} />
+        </div>
+
+        {/* Auto-Matching Configuration */}
+        <div className="mt-6">
+          <MatchingConfigSection slug={slug} event={event} onRefresh={fetchEventDetails} />
+        </div>
+
+        {/* Matching Runs (admin tools) */}
+        <div className="mt-6">
+          <MatchingRunsSection slug={slug} />
+        </div>
+
+        {/* Schedule Configuration */}
+        <div className="mt-6">
+          <ScheduleConfigSection slug={slug} event={event} onRefresh={fetchEventDetails} />
+        </div>
+
+        {/* Action Buttons */}
+        <div className="mt-6 flex gap-4">
+          <Link
+            to={`/events/${slug}/chat`}
+            className="flex-1 bg-primary-600 text-white px-6 py-3 rounded-lg hover:bg-primary-700 transition-colors text-center font-medium"
+          >
+            Go to Event Chat
+          </Link>
+          <button
+            onClick={() => window.print()}
+            className="px-6 py-3 border border-gray-300 rounded-lg hover:bg-gray-50 transition-colors font-medium"
+          >
+            Print QR Code
+          </button>
+        </div>
+
+        {/* Print Styles */}
+        <style>{`
+          @media print {
+            body * {
+              visibility: hidden;
+            }
+            .print-area, .print-area * {
+              visibility: visible;
+            }
+            .print-area {
+              position: absolute;
+              left: 0;
+              top: 0;
+            }
+          }
+        `}</style>
+      </div>
+    </Layout>
+  );
+}