export async function joinRoom(roomId: string, userId: string): Promise<void> {
  const res = await fetch(`/api/room/${roomId}/join`, {
    method: "POST",
    headers: { "Content-Type": "application/json" },
    body: JSON.stringify({ userId }),
  });
  if (!res.ok) {
    const data = await res.json().catch(() => ({}));
    throw new Error(data.error || "加入房间失败");
  }
}