dsh-skin-koi-pond: Koi Pond Theme

`dsh-skin-koi-pond` is a presentation-only theme plugin for the DeepSeek Harness WebUI, delivering a "koi pond" visual experience. It uses Canvas 2D to render a koi schooling animation, with mouse interaction and light/dark theme support. It overrides official design tokens using a seven-part CSS structure, without interfering with underlying logic or services. After installation, color schemes can be switched in real time via console APIs, and custom builds are supported.