dsh-bg-skin: DSH UI Background Skin Plugin

`dsh-bg-skin` is a DeepSeek Harness (DSH)-based plugin designed to replace the default white background with a local image. It offers a default skin, a custom image path, and an automatic fallback mechanism to ensure the interface does not show white gaps and that text remains legible. It relies on minimal dependencies, supports real-time updates, is easy to install, and includes fallback background colors for light and dark modes.