fix: hide navigation tabs completely for non-admin users

src/App.js

@@ -1477,10 +1477,13 @@ function App() {
 
 function NavigationTabs({ isAdmin }) {
   const location = useLocation();
-  const tabs = [{ to: '/', label: 'Konfiguration' }];
-  if (isAdmin) {
-    tabs.push({ to: '/admin', label: 'Admin' });
+  if (!isAdmin) {
+    return null;
   }
+  const tabs = [
+    { to: '/', label: 'Konfiguration' },
+    { to: '/admin', label: 'Admin' }
+  ];
 
   return (
     <div className="flex flex-wrap gap-2 mb-4">