feat: replace native confirm() dialogs with custom modal component

- Add ModalProvider context and useConfirm hook (confirm-modal.tsx)
- Support danger/warning/info variants with icons and colors
- Animated backdrop and dialog with CSS keyframes
- Accessible: aria-modal, role=dialog, ESC to close, auto-focus
- Replace all 6 native confirm() calls across dashboard pages
- Integrate ModalProvider in app providers

frontend/src/app/dashboard/admin/pools/page.tsx

@@ -6,9 +6,11 @@ import api from '@/lib/api';
 import { toast } from 'react-toastify';
 import type { Cluster, ClusterPool } from '@/types';
 import { Scale, BarChart3, RotateCw, CheckCircle, XCircle, Pencil, Clock, X } from 'lucide-react';
+import { useConfirm } from '@/components/confirm-modal';
 
 export default function AdminPoolsPage() {
   const queryClient = useQueryClient();
+  const confirm = useConfirm();
   const [showForm, setShowForm] = useState(false);
   const [editingPool, setEditingPool] = useState<ClusterPool | null>(null);
   const [form, setForm] = useState({
@@ -318,10 +320,14 @@ export default function AdminPoolsPage() {
                       <Pencil className="w-3 h-3 inline" /> Edit
                     </button>
                     <button
-                      onClick={() => {
-                        if (confirm(`Delete pool "${pool.name}"? Apps already assigned to this pool will keep their current cluster.`)) {
-                          deleteMutation.mutate(pool.id);
-                        }
+                      onClick={async () => {
+                        const ok = await confirm({
+                          title: `Delete Pool "${pool.name}"`,
+                          message: 'Apps already assigned to this pool will keep their current cluster.',
+                          confirmText: 'Delete',
+                          variant: 'danger',
+                        });
+                        if (ok) deleteMutation.mutate(pool.id);
                       }}
                       className="text-sm text-red-600 hover:text-red-800"
                     >