diff --git a/audit.md b/audit.md
index be5d494..21d31a9 100644
--- a/audit.md
+++ b/audit.md
@@ -74,6 +74,11 @@ Verify mutations `invalidateQueries`; consider optimistic insert so reviews appe
 ---
 
 ## Implementation status
+- [x] Q2 — `useCategories()` query hook (`api/queries/useCategories.ts`, key
+  `queryKeys.categories()`, 30 min `staleTime`). Replaced raw
+  `api.get('categories/')` + `useState`/`useEffect` in `App.tsx` with the hook;
+  dropped the now-unused `api` import. Cached + shared across mounts. `tsc` +
+  ESLint (0 errors) clean.
 - [x] R1 + R2 + Q1 — notif menu dedup + i18n + SPA nav. Extracted shared
   `<NotificationMenu>` (`components/layout/NotificationMenu.tsx`) + `useNotificationClick()`
   hook (`hooks/useNotificationClick.ts`); `App.tsx` and `NavSidebar.tsx` now both render
diff --git a/frontend/src/App.tsx b/frontend/src/App.tsx
index 267d307..aa9fbef 100644
--- a/frontend/src/App.tsx
+++ b/frontend/src/App.tsx
@@ -16,7 +16,6 @@ import { GlassAppBar } from './components/ui';
 import MenuIcon from '@mui/icons-material/Menu';
 import NotificationsIcon from '@mui/icons-material/Notifications';
 import { useTranslation } from 'react-i18next';
-import api from './lib/api';
 import Footer from './components/Footer';
 import CookieBanner from './components/CookieBanner';
 import { GlobalSearch } from './app/GlobalSearch';
@@ -24,6 +23,7 @@ import { useAuth } from './hooks/useAuth';
 import { useTheme } from './hooks/useTheme';
 import { useDrawerAnchor } from './hooks/useDrawerAnchor';
 import { NavSidebar, NAV_SIDEBAR_WIDTH } from './components/layout/NavSidebar';
+import { useCategories } from './api/queries/useCategories';
 import { NotificationMenu } from './components/layout/NotificationMenu';
 import { RequireSuperuser } from './components/auth/RequireSuperuser';
 import type { CatppuccinTheme } from './theme';
@@ -61,20 +61,10 @@ const App: React.FC = () => {
   const { anchor: mobileAnchor } = useDrawerAnchor();
 
   const [notifAnchorEl, setNotifAnchorEl] = useState<null | HTMLElement>(null);
-  const [categories, setCategories] = useState<{ name: string; slug: string }[]>([]);
+  const { data: categories = [] } = useCategories();
   const isMobile = useMediaQuery(muiTheme.breakpoints.down('md'));
   const [drawerOpen, setDrawerOpen] = useState(false);
 
-  useEffect(() => {
-    api
-      .get('categories/')
-      .then((res) => {
-        const data = Array.isArray(res.data) ? res.data : res.data.results || [];
-        setCategories(data);
-      })
-      .catch(() => setCategories([]));
-  }, []);
-
   useEffect(() => {
     document.body.style.backgroundColor = muiTheme.palette.background.default;
     document.body.style.color = muiTheme.palette.text.primary;
diff --git a/frontend/src/api/keys.ts b/frontend/src/api/keys.ts
index c4b48af..28d7164 100644
--- a/frontend/src/api/keys.ts
+++ b/frontend/src/api/keys.ts
@@ -4,6 +4,7 @@
  * params (URL-stable, not the full args object).
  */
 export const queryKeys = {
+  categories: () => ['categories'] as const,
   dashboard: () => ['dashboard'] as const,
   publicProfile: (username: string) => ['publicProfile', username] as const,
   publicProfileAll: () => ['publicProfile'] as const,
diff --git a/frontend/src/api/queries/useCategories.ts b/frontend/src/api/queries/useCategories.ts
new file mode 100644
index 0000000..8bc0431
--- /dev/null
+++ b/frontend/src/api/queries/useCategories.ts
@@ -0,0 +1,22 @@
+import { useQuery } from '@tanstack/react-query';
+import api from '../../lib/api';
+import { queryKeys } from '../keys';
+
+export interface Category {
+  name: string;
+  slug: string;
+}
+
+/**
+ * Nav categories. Rarely change, so cached aggressively — one fetch shared by
+ * the AppBar drawer and desktop sidebar instead of a raw `useEffect` per mount.
+ */
+export const useCategories = () =>
+  useQuery({
+    queryKey: queryKeys.categories(),
+    queryFn: async () => {
+      const res = await api.get<Category[] | { results: Category[] }>('categories/');
+      return Array.isArray(res.data) ? res.data : (res.data.results ?? []);
+    },
+    staleTime: 30 * 60_000,
+  });