feat: implement Portal component and refactor modals to use it for improved positioning

assets/admin/pages/PreRegistrationsPage.tsx

@@ -4,6 +4,7 @@ import { CheckIcon, XMarkIcon, PhoneIcon } from '@heroicons/react/24/outline';
 import { toast } from 'sonner';
 import { api } from '../lib/api';
 import type { PaginatedResponse } from '../lib/api';
+import Portal from '../components/ui/Portal';
 import { formatDate } from '../lib/utils';
 import PageHeader from '../components/ui/PageHeader';
 import Pagination from '../components/ui/Pagination';
@@ -204,6 +205,7 @@ export default function PreRegistrationsPage() {
 
       {/* Reject Dialog */}
       {rejectTarget && (
+        <Portal>
         <div className="overlay" onClick={() => setRejectTarget(null)}>
           <div className="modal" style={{ maxWidth: 420 }} onClick={(e) => e.stopPropagation()}>
             <div className="modal-head">
@@ -232,6 +234,7 @@ export default function PreRegistrationsPage() {
             </div>
           </div>
         </div>
+        </Portal>
       )}
     </div>
   );