dsh-shutdown-button: DeepSeek Harness Graceful Shutdown Plugin

dsh-shutdown-button is a client plugin maintained by Yelloooooow, designed to address the Fiber thread leak issue caused by directly closing the terminal in DeepSeek Harness (DSH). By adding a shutdown button at the bottom of the Web GUI sidebar, it provides a graphical entry point for graceful exit. The plugin features a confirmation dialog, expand/collapse interaction, orderly shutdown, persistence and services, and automatic loading. Users simply need to click the power icon in the bottom-left corner and confirm, which triggers `ctx.appExit` and ensures the process exits normally with exit code 0. The installation command is `dsh plugin --profile web add github:Yelloooooow/dsh-shutdown-button`, and the uninstall command is `dsh plugin --profile web remove github:Yelloooooow/dsh-shutdown-button`.