dsh-bgwall-plugin: Replace DSH Web interface background with image

`dsh-bgwall-plugin` is a pure JavaScript-based DeepSeek Harness (DSH) plugin designed to address the issue of non-customizable Web interface backgrounds. This plugin allows users to replace backgrounds with remote URLs or local server files and provides real-time adjustment controls for transparency, blur, and tiling modes (cover/fit/stretch). It also supports sidebar background transparency settings and one-click background clearing. To install, add the plugin from GitHub using the `dsh plugin` command, then restart the Web profile and refresh the browser to access the "Background Image" control panel in the "Settings" menu. For local files, relative paths are resolved based on the workspace root directory, with a maximum file size limit of 15MB per image. This plugin is licensed under the MIT License.