dsh-show-media: Show Local Media in DeepSeek Harness Chat Cards

`dsh-show-media` is a client-side plugin for DeepSeek Harness (DSH) designed to address the issue that local media files cannot be rendered directly. By defining the `dsh_show_media` tool, it allows users to display local images (supporting PNG, JPEG, WebP, and GIF) and videos (supporting MP4, WebM, and others, up to 64MiB) directly in a conversation. Installation requires using the `dsh plugin` command, and once enabled, it is invoked by passing a JSON-formatted `file_path` parameter. Its functionality applies only to the current conversation card, supports click-to-preview and closing with ESC, is primarily intended for human observers, and does not replace the official model vision pathway.