dsh-quote-selection: Insert selected conversation text as a quote in the editor

The DeepSeek Harness (DSH) Web UI plugin `lianginx/dsh-quote-selection` allows users to select any text in a conversation and click a floating button to insert it into the editor as a Markdown blockquote. It does not rely on build tools, is implemented in pure native JavaScript, and supports automatic language adaptation, chained citations, and zero-build installation. It addresses the tedious and error-prone process of manual copy-pasting and is suitable for scenarios involving follow-up questions or summaries based on earlier paragraphs.