fix: add index redirect for /account route

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

frontend/src/router.tsx

@@ -216,6 +216,10 @@ export const router = createBrowserRouter([
           </Suspense>
         ),
         children: [
+          {
+            index: true,
+            element: <Navigate to="/account/categories" replace />,
+          },
           {
             path: 'categories',
             element: (