dsh-minimap: Add a VS Code-Style Thumbnail to the DSH Web GUI Sidebar

`dsh-minimap` is a plugin for the DSH Web interface, designed to address the low efficiency of browsing long text files. By overlaying a miniature text thumbnail on the edge of the sidebar text editor, it delivers a navigation experience similar to VS Code. The plugin supports click-to-jump, dragging the viewport frame to reposition, and hover-scroll operations, while featuring syntax highlighting and adaptive layout. It uses a pure overlay architecture, operates independently across multiple tabs, and is fully compatible with dsh-better-sidebar. Users can install it using the `dsh plugin` command; after installation, the Web service must be restarted.