cjz-wr/background-plugin: Add background settings for DeepSeek Harness

`cjz-wr/background-plugin` is an MIT-licensed independent plugin for DeepSeek Harness (DSH), designed to extend background setting capabilities for DSH without replacing the core program. The plugin supports setting local or online images and videos as backgrounds, allowing users to adjust transparency and persistently save preferences. Users can search for and install the plugin through DSH's Web plugin interface or use the CLI command `dsh plugin --profile <name> add github:cjz-wr/background-plugin`; the new plugin will take effect on the next process startup. The project's build artifacts have been directly committed to the repository and are usable without executing the build script; if modifying source code, run `npm...