dsh-customui-plugin: Add a "Personalization" settings page for DSH Web

# Introduction In the DSH (DeepSeek Harness) web version, several interface elements are hardcoded into the host build artifacts: the brand logo in the top-left corner of the sidebar, the welcome area during empty conversations, and the background color of the conversation area. If you wish to replace them with your own logos or background images in an on-premises deployment, the conventional approach is to modify the host build, which requires redoing the work every time the host is updated. dsh-customui-plugin takes a different approach: it avoids modifying the host build and instead injects scripts into the browser via the plugin mechanism to overlay changes at the DOM layer, while also providing a formal entry point for these modifications in the settings page. The following sections introduce its features, working principles, and installation steps. # What is it? dsh-customui-plugin is a DSH web platform plugin, with the repository address at https://g