dsh-markdown-preview: Real-Time Markdown and KaTeX Preview for DSH Web Plugin

`dsh-markdown-preview` is a DeepSeek Harness (DSH) Web plugin designed to solve the lack of immediate Markdown feedback in the input box. It leverages the official Slot to provide a dedicated real-time preview panel above the input field, supporting bold text, formulas (KaTeX), and rendering of multiple syntax types. Its core features include a pinned layout, interactive collapse/expand, a content-escaping security mechanism, and seamless compatibility with existing functionality. Plugin installation supports both an official command and local mounting, and the plugin is ready to use after a restart, offering a WYSIWYG input experience.