debug: add error boundary to show actual error message

client/src/app/error.tsx

@@ -0,0 +1,17 @@
+"use client";
+
+import { useEffect } from "react";
+
+export default function Error({ error, reset }: { error: Error & { digest?: string }; reset: () => void }) {
+  useEffect(() => {
+    console.error("App error:", error);
+  }, [error]);
+
+  return (
+    <div className="flex flex-col items-center gap-4 py-20">
+      <h2 className="text-xl font-bold">Fehler aufgetreten</h2>
+      <pre className="bg-base-200 rounded p-4 text-sm max-w-xl overflow-auto">{error.message}</pre>
+      <button className="btn btn-primary" onClick={reset}>Neu laden</button>
+    </div>
+  );
+}