dsh-restart-systemd: Add a restart button with automatic session reconnection to the DSH WebUI

If you run dsh-web using `systemctl --user`, you are likely familiar with this workflow: opening a terminal, executing `systemctl --user restart dsh-web`, and then manually refreshing the page in your browser. If an agent task is running, that conversation turn is interrupted, requiring you to manually type a command to resume it. `dsh-restart-systemd` brings this functionality into the WebUI: simply click the restart button in the sidebar (or type the `/restart` command), the service will restart in approximately 3 seconds, the browser will reconnect automatically, and the interrupted session will resume seamlessly. The following sections introduce its implementation logic, installation methods, and usage limitations.