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

frontend/src/stream/useSessionStream.ts

@@ -0,0 +1,31 @@
+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 };
+}