Add global admin user-view preview

frontend/app/ui/UserViewBanner.tsx

@@ -0,0 +1,21 @@
+'use client'
+
+import { setUserViewPreview, useUserViewPreview } from '../lib/viewMode'
+
+export default function UserViewBanner() {
+  const enabled = useUserViewPreview()
+
+  if (!enabled) return null
+
+  return (
+    <div className="user-view-banner" role="status">
+      <div>
+        <strong>User view</strong>
+        <span>You are previewing the non-admin experience. Your account and backend permissions remain admin.</span>
+      </div>
+      <button type="button" onClick={() => setUserViewPreview(false)}>
+        Exit user view
+      </button>
+    </div>
+  )
+}