feat(frontend): widget registry + universal fallback

frontend/src/widgets/FallbackWidget.tsx

@@ -0,0 +1,14 @@
+import { useState } from "react";
+import type { WidgetProps } from "./types";
+
+export function FallbackWidget({ descriptor, onRespond }: WidgetProps) {
+  const [text, setText] = useState("");
+  return (
+    <div className="border rounded p-3 space-y-2">
+      <p className="text-sm text-amber-600">Widget non supportato (kind: {descriptor.widget}) — rispondi manualmente</p>
+      <pre className="text-xs overflow-auto max-h-48 bg-muted p-2">{JSON.stringify(descriptor, null, 2)}</pre>
+      <textarea className="w-full border rounded p-1" value={text} onChange={(e) => setText(e.target.value)} />
+      <button className="border rounded px-2 py-1" onClick={() => onRespond({ id: descriptor.id, control: "freetext", text })}>Invia</button>
+    </div>
+  );
+}