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/App.jsx

@@ -10,7 +10,7 @@ import ResetPasswordPage from './pages/ResetPasswordPage';
 import DashboardPage from './pages/DashboardPage';
 import EventsPage from './pages/EventsPage';
 import EventChatPage from './pages/EventChatPage';
-import EventDetailsPage from './pages/EventDetailsPage';
+import EventDetailsPage from './pages/admin/EventDetailsPage';
 import EventCheckinPage from './pages/EventCheckinPage';
 import MatchChatPage from './pages/MatchChatPage';
 import MatchesPage from './pages/MatchesPage';
@@ -144,7 +144,7 @@ function App() {
             }
           />
           <Route
-            path="/events/:slug/details"
+            path="/admin/events/:slug/details"
             element={
               <ProtectedRoute>
                 <EventDetailsPage />