fix(c5,arch-021): system dashboard nav entry only for system admins

frontend/src/components/layout/Sidebar.tsx

@@ -79,7 +79,11 @@ export function Sidebar() {
   };
 
   const allMenuItems = useMemo(() => {
-    const staticItems = singleItems.map(item => ({
+    const staticItems = singleItems
+      // Backend enforces require_admin on system dashboard routes — hide the
+      // nav entry from non-admin users instead of showing a dead link.
+      .filter(item => item.to !== '/system-dashboard' || user?.is_system_admin)
+      .map(item => ({
         path: item.to,
         labelKey: item.labelKey,
         label: item.labelKey,