feat(frontend): widget registry + universal fallback

frontend/src/widgets/FallbackWidget.tsx

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+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>
+  );
+}

frontend/src/widgets/registry.test.tsx

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+import { render, screen } from "@testing-library/react";
+import { register, resolve } from "./registry";
+import type { WidgetProps } from "./types";
+
+test("resolve returns the registered renderer", () => {
+  const Dummy = (_: WidgetProps) => <div>dummy</div>;
+  register("dummy", Dummy);
+  expect(resolve("dummy")).toBe(Dummy);
+});
+
+test("resolve falls back for unknown kind and shows the JSON", () => {
+  const Comp = resolve("totally-unknown");
+  render(<Comp descriptor={{ id: "u1", widget: "totally-unknown", title: "X" } as any} onRespond={() => {}} />);
+  expect(screen.getByText(/Widget non supportato/i)).toBeInTheDocument();
+});

frontend/src/widgets/registry.ts

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+import type React from "react";
+import type { WidgetProps } from "./types";
+import { FallbackWidget } from "./FallbackWidget";
+
+const registry = new Map<string, React.FC<WidgetProps>>();
+
+export function register(kind: string, comp: React.FC<WidgetProps>) {
+  registry.set(kind, comp);
+}
+
+export function resolve(kind: string): React.FC<WidgetProps> {
+  return registry.get(kind) ?? FallbackWidget;
+}

frontend/src/widgets/types.ts

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+import type { WidgetDescriptor, UiResponse } from "../api/types";
+
+export type WidgetProps = { descriptor: WidgetDescriptor; onRespond: (r: UiResponse) => void };