dsh-chat-offscreen: Keeping Long Conversations Responsive

The DeepSeek Harness (DSH) Web chat session does not virtualize chat history, so the excessive number of DOM nodes causes UI lag during long conversations. The `dsh-chat-offscreen` plugin injects a stylesheet and uses the CSS `content-visibility` property to skip layout and painting for offscreen areas, providing a low-overhead approximation to virtualization, while using `contain-intrinsic-size` to keep the scroll range stable. This solution is compatible with Chrome/Edge 85+, Firefox 125+, and Safari 18+, and can be used together with the retired P0 rule as a temporary workaround until official DSH virtualization support is available.