Reduce retry count and show errors on admin pages

TanStack Query's default 3 retries + exponential backoff hid backend
errors behind 5-8s of "Loading…". Now retries once and surfaces the
error message immediately on failure.

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

frontend/src/pages/AdminGroupsPage.tsx

@@ -15,9 +15,10 @@ import {
 
 export default function AdminGroupsPage() {
   const queryClient = useQueryClient();
-  const { data: groups = [], isLoading } = useQuery({
+  const { data: groups = [], isLoading, isError, error } = useQuery({
     queryKey: ["admin-groups"],
     queryFn: adminListGroups,
+    retry: 1,
   });
 
   const [showForm, setShowForm] = useState(false);
@@ -97,6 +98,11 @@ export default function AdminGroupsPage() {
 
       {isLoading ? (
         <p>Loading…</p>
+      ) : isError ? (
+        <p style={{ color: "red" }}>
+          Failed to load groups:{" "}
+          {(error as any)?.response?.data?.detail ?? (error as any)?.message ?? "Unknown error"}
+        </p>
       ) : (
         <table style={{ width: "100%", borderCollapse: "collapse", marginBottom: 24 }}>
           <thead>

frontend/src/pages/AdminUsersPage.tsx

@@ -13,9 +13,10 @@ import {
 export default function AdminUsersPage() {
   const queryClient = useQueryClient();
   const { data: me } = useQuery({ queryKey: ["me"], queryFn: getMe });
-  const { data: users = [], isLoading } = useQuery({
+  const { data: users = [], isLoading, isError, error } = useQuery({
     queryKey: ["admin-users"],
     queryFn: adminGetUsers,
+    retry: 1,
   });
 
   const [showForm, setShowForm] = useState(false);
@@ -71,6 +72,11 @@ export default function AdminUsersPage() {
 
       {isLoading ? (
         <p>Loading…</p>
+      ) : isError ? (
+        <p style={{ color: "red" }}>
+          Failed to load users:{" "}
+          {(error as any)?.response?.data?.detail ?? (error as any)?.message ?? "Unknown error"}
+        </p>
       ) : (
         <table style={{ width: "100%", borderCollapse: "collapse", marginBottom: 24 }}>
           <thead>