Multi-Device Co-op Mode for UTAGE or Another Charts.

**Is your feature request related to a problem? Please describe.**
When attempting to play custom UTAGE charts that require collaboration on two devices, or when trying to simulate a synchronized co-op gaming experience, I find it challenging to start the game simultaneously on both devices.

**Describe the solution you'd like**
I propose a feature that allows multiple devices to connect via Wi-Fi or Bluetooth. With this connection, once all devices are ready and the play button is pressed, the chart will start playing simultaneously on all devices. The specific steps are as follows:

1. Players create or join a room.
2. The host selects the chart, and other devices receive the chart file from the host’s device via the local network.
3. Wait for all players to press the ready button.
4. After all players are ready, the game emits a prompt sound and starts simultaneously on all devices after three seconds.


该 issue 描述了一个需要网络同步的多设备 co-op 模式。研究项目中现有的 networking 代码，可能是使用 Unity、Photon、Mirror 或自定义方案。查找 chart 加载和游戏开始逻辑。原型需要 room 创建、通过本地网络传输 chart 文件，以及同步的倒计时开始。

由索引模型根据 Issue 内容生成。