dsh-web-terminal: Add a multi-tab local terminal to DSH web GUI

# Introduction When developing agents using DeepSeek Harness (DSH), most of the time is spent in the web GUI. However, whenever you need to execute shell commands—such as starting services, viewing logs, or temporarily entering vim to modify configurations—you have to switch to a separate terminal window and manually reconcile the directory and session state upon returning. `dsh-web-terminal` solves this problem: it adds a Terminal entry to the web GUI sidebar, opening a multi-tab terminal panel where each tab is a real PTY shell on the host process. DSH's philosophy is that "everything is a plugin"; therefore, this plugin requires no modification of DSH source code and can be installed directly into the web profile. The following sections cover the features, installation, configuration, and verification in order.