feat(frontend): useSessionStream (EventSource -> store)

frontend/src/stream/useSessionStream.test.tsx

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+import { renderHook } from "@testing-library/react";
+import { act } from "react";
+import { FakeEventSource } from "../test/fakeEventSource";
+import { useSessionStream } from "./useSessionStream";
+import { useSessionStore } from "../store/sessionStore";
+
+beforeEach(() => {
+  FakeEventSource.instances = [];
+  (globalThis as any).EventSource = FakeEventSource;
+  useSessionStore.getState().resetSession();
+});
+
+test("opens an EventSource for the session and feeds events to the store", () => {
+  renderHook(() => useSessionStream("s1"));
+  const es = FakeEventSource.instances[0];
+  expect(es.url).toContain("/sessions/s1/events");
+  act(() =>
+    es.emit({ type: "ui_request", ui_request: { id: "u1", widget: "select" } })
+  );
+  expect(useSessionStore.getState().pendingWidget?.id).toBe("u1");
+});
+
+test("closes the stream on unmount", () => {
+  const { unmount } = renderHook(() => useSessionStream("s1"));
+  const es = FakeEventSource.instances[0];
+  unmount();
+  expect(es.closed).toBe(true);
+});

frontend/src/stream/useSessionStream.ts

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+import { useEffect, useState } from "react";
+import { BASE } from "../api/client";
+import { useSessionStore } from "../store/sessionStore";
+import type { StreamEvent } from "../api/types";
+
+export function useSessionStream(sessionId: string | null) {
+  const [connected, setConnected] = useState(false);
+  const applyEvent = useSessionStore((s) => s.applyEvent);
+
+  useEffect(() => {
+    if (!sessionId) return;
+
+    const es = new EventSource(`${BASE}/sessions/${sessionId}/events`);
+    es.onopen = () => setConnected(true);
+    es.onerror = () => setConnected(false);
+    es.onmessage = (ev) => {
+      try {
+        applyEvent(JSON.parse(ev.data) as StreamEvent);
+      } catch {
+        /* ignore malformed */
+      }
+    };
+
+    return () => {
+      es.close();
+      setConnected(false);
+    };
+  }, [sessionId, applyEvent]);
+
+  return { connected };
+}

frontend/src/test/fakeEventSource.ts

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+export class FakeEventSource {
+  static instances: FakeEventSource[] = [];
+  onmessage: ((e: { data: string }) => void) | null = null;
+  onopen: (() => void) | null = null;
+  onerror: (() => void) | null = null;
+  closed = false;
+  constructor(public url: string) {
+    FakeEventSource.instances.push(this);
+  }
+  emit(obj: unknown) {
+    this.onmessage?.({ data: JSON.stringify(obj) });
+  }
+  close() {
+    this.closed = true;
+  }
+}