dsh-plugin-terminal: Use real PTY terminal in DSH Web bottom panel

`dsh-plugin-terminal` is a plugin for DSH Web, developed by `mervyn-teo` and licensed under the MIT License. It is based on host PTY, WebSocket, and xterm.js to provide a real interactive PTY terminal within a VS Code-style collapsible bottom panel. The plugin supports ANSI colors, cursor movement, and 256-color rendering, making it suitable for default shells in Windows PowerShell and other environments. Users can toggle the panel using the `Ctrl+`` ` shortcut, adjust the height, and customize settings such as theme, font, and working directory; some settings take effect immediately, while shell changes are applied on the next startup. Installation requires ensuring that DSH mounts the relevant services.