dsh-plugin-gemini-theme: Give the DeepSeek Harness Web Client a Gemini Skin

The dsh-plugin-gemini-theme plugin for DeepSeek Harness (DSH) overrides the `--dsw-*` design tokens via a single stylesheet, allowing the Web client interface to be customized into a Google Gemini-like style without modifying the source code. It mainly resets styles for the chat area, sidebar, input box, animations, and glow effects. Its styles have extremely high specificity and do not interfere with the DOM structure. Developers must use specific commands to install and remove it, and should be aware of the risk of unintended style matches caused by class-name matching. Additionally, the session header, native carousel, and similar elements are not covered.