dsh-bg-plugin: Add Custom Background Images and Videos to DSH

The DeepSeek Harness (DSH) plugin dsh-bg-plugin allows users to customize the interface background using remote URLs or local files (images and videos are supported), while providing adjustable semi-transparent overlays and clarity settings. Its key features include: forces the settings panel to retain its default appearance; supports previewing selected image regions; and provides a floating ball panel background patch with bidirectional synchronization. Installation is available in both static (recommended and persistent) and dynamic modes, with configurations automatically saved to a local state file. The plugin supports automatic updates and includes separate installation guides and file permission notes for desktop and web interfaces.