fix: subscribe to model activity events

frontend/src/stream/useSessionStream.test.tsx

@@ -22,6 +22,17 @@ test("opens an EventSource and feeds NAMED events to the store", () => {
   expect(useSessionStore.getState().pendingWidget?.id).toBe("u1");
 });
 
+test("feeds named activity_delta events to the model activity store", () => {
+  renderHook(() => useSessionStream("s1"));
+  const es = FakeEventSource.instances[0];
+
+  act(() =>
+    es.emitNamed("activity_delta", { type: "activity_delta", text: "Analizzo le ambiguità" })
+  );
+
+  expect(useSessionStore.getState().activity.at(-1)?.text).toBe("Analizzo le ambiguità");
+});
+
 test("also feeds UNNAMED (default message) events to the store", () => {
   renderHook(() => useSessionStream("s1"));
   const es = FakeEventSource.instances[0];

frontend/src/stream/useSessionStream.ts

@@ -26,7 +26,13 @@ export function useSessionStream(sessionId: string | null) {
     // onmessage catches unnamed events; addEventListener catches named events
     // (the backend sends event: ui_request, event: text_delta, etc.)
     es.onmessage = handle;
-    const namedEvents = ["ui_request", "text_delta", "info", "system_event"] as const;
+    const namedEvents = [
+      "ui_request",
+      "text_delta",
+      "activity_delta",
+      "info",
+      "system_event",
+    ] as const;
     for (const name of namedEvents) es.addEventListener(name, handle);
 
     return () => {