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/pages/admin/AdminRuns.tsx

@@ -120,8 +120,9 @@ export function AdminRuns() {
               onSuccess: () => setDeleting(null),
             })
           }
-          onCancel={() => setDeleting(null)}
+          onCancel={() => { setDeleting(null); deleteRun.reset() }}
           isDeleting={deleteRun.isPending}
+          error={deleteRun.error?.message ?? null}
         />
       )}
     </div>