fix: show manage controls for system categories when user is superuser

canManage() returned false for system-scope categories unconditionally.
Superusers can manage all categories (backend already permits it), so
check is_admin from getMe() and short-circuit to true.

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

frontend/src/components/ManageCategoriesDialog.tsx

@@ -6,6 +6,7 @@ import {
   renameCategory,
   deleteCategory,
   getMyGroups,
+  getMe,
   type CategoryOut,
   ApiError,
 } from "@/api/client";
@@ -29,6 +30,9 @@ export default function ManageCategoriesDialog({ onClose }: Props) {
     queryFn: getMyGroups,
   });
 
+  const { data: me } = useQuery({ queryKey: ["me"], queryFn: getMe });
+  const isSuperuser = me?.is_admin ?? false;
+
   // Set of group IDs for which the current user is a group admin
   const adminGroupIds = new Set(myGroups.filter((g) => g.is_group_admin).map((g) => g.id));
 
@@ -60,9 +64,10 @@ export default function ManageCategoriesDialog({ onClose }: Props) {
   });
 
   function canManage(cat: CategoryOut): boolean {
+    if (isSuperuser) return true;
     if (cat.scope === "personal") return true;
     if (cat.scope === "group") return cat.group_id != null && adminGroupIds.has(cat.group_id);
-    return false; // system — managed only by superuser via other means
+    return false; // system — non-admin cannot manage
   }
 
   const filtered = search