Show error feedback when run deletion is blocked

Add optional error prop to DeleteConfirmModal and wire it into AdminRuns
so the backend's rejection message is displayed to the user.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/components/admin/DeleteConfirmModal.tsx

@@ -4,6 +4,7 @@ interface DeleteConfirmModalProps {
   onConfirm: () => void
   onCancel: () => void
   isDeleting?: boolean
+  error?: string | null
 }
 
 export function DeleteConfirmModal({
@@ -12,6 +13,7 @@ export function DeleteConfirmModal({
   onConfirm,
   onCancel,
   isDeleting,
+  error,
 }: DeleteConfirmModalProps) {
   return (
     <div className="fixed inset-0 z-50 flex items-center justify-center">
@@ -24,6 +26,11 @@ export function DeleteConfirmModal({
           <p className="mt-2 text-sm text-gray-600 dark:text-gray-300">
             {message}
           </p>
+          {error && (
+            <p className="mt-2 text-sm text-red-600 dark:text-red-400">
+              {error}
+            </p>
+          )}
         </div>
         <div className="px-6 py-4 border-t border-gray-200 dark:border-gray-700 flex justify-end gap-3">
           <button