dsh-wx-skin: Add skin background to DSH Web GUI

`dsh-wx-skin` is a skin plugin for DSH Web GUI developed by wangxilhy23 and licensed under the MIT License. It allows users to customize backgrounds via a "Skin" panel in the sidebar, supporting local uploads of PNG, JPEG, WebP, GIF, and BMP images, as well as pasting HTTP(S) image URLs. The plugin also provides six built-in presets, such as Ink Blue and Aurora Gradients. It offers adjustment controls for darkening (0-80%), blurring (0-24px), and opacity (0-100%), and supports automatic adaptation to both light and dark themes. Settings are persisted via localStorage and configuration files, automatically restoring after page refreshes or application restarts. The plugin operates as an external client and does not modify the DSH main repository. Installation requires Node.js and pnpm.