修复一个会让整个应用失去响应的严重问题。

🐛 Bug 修复

修复应用长时间运行（尤其经历睡眠唤醒）后，第一次听写让整个应用卡住、只能强制退出的问题。旧录音设备在系统音频服务异常时无法在主线程上正常关闭，把整个应用一起拖住。现在设备关闭改在后台进行：即使系统音频再异常，也只会影响当次听写——15 秒后会提示「启动超时」，可以直接重试，应用本身始终可用。

v2.1.5: No more freeze after long idle

Fixes a serious issue that could make the whole app unresponsive.

Fixed the first dictation after a long uptime (especially across sleep/wake) freezing the entire app so it had to be force-quit. The previous recording device could not shut down on the main thread while the system audio service was stuck, taking the whole app down with it. Device shutdown now happens in the background: even if the system audio service misbehaves again, only that one dictation is affected — after 15 seconds you get a "start timed out" notice and can simply retry, while the app itself stays responsive.