dsh-cad-viewer: 3D Model Library and CAD Viewer Plugin for DeepSeek Harness

The DSH plugin `dsh-cad-viewer` adds a 3D model library and CAD viewing capabilities to the DeepSeek Harness Web GUI. It supports generating models via CadQuery scripts and adding them to the library, and provides model management, online viewing, and export in 10 formats (STEP, STL, SVG, etc.). Models are stored server-side and shared across devices on the same instance. A service restart is required after installation, and operations include using tools to build, query, and delete models.