feat: add admin settings page with toggle switch for demo banner

shop/app/admin/einstellungen/page.tsx

@@ -0,0 +1,33 @@
+'use client';
+
+import { useState, useEffect } from 'react';
+import { Switch } from '@/components/ui/switch';
+
+export default function AdminSettings() {
+  const [demoActive, setDemoActive] = useState(true);
+
+  useEffect(() => {
+    const state = localStorage.getItem('demo_banner_disabled') !== 'true';
+    setDemoActive(state);
+  }, []);
+
+  const toggleDemo = (checked: boolean) => {
+    setDemoActive(checked);
+    localStorage.setItem('demo_banner_disabled', (!checked).toString());
+    // Event feuern, damit andere Tabs/Komponenten sofort updaten
+    window.dispatchEvent(new Event('storage_demo_changed'));
+  };
+
+  return (
+    <div className="p-6 max-w-xl mx-auto text-slate-900 dark:text-white">
+      <h1 className="text-2xl font-bold mb-6">Admin Einstellungen</h1>
+      <div className="flex items-center justify-between p-4 bg-slate-100 dark:bg-white/5 rounded-xl border border-slate-200 dark:border-white/10">
+        <div>
+          <p className="font-medium text-slate-900 dark:text-white">Demo-Warnmeldungen</p>
+          <p className="text-xs text-slate-500 dark:text-slate-400">Schaltet gelbe Banner ein oder aus.</p>
+        </div>
+        <Switch checked={demoActive} onCheckedChange={toggleDemo} />
+      </div>
+    </div>
+  );
+}