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

frontend/src/test/fakeEventSource.ts

@@ -0,0 +1,16 @@
+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;
+  }
+}