dsh-message-navigator: Message navigation bar for the DeepSeek Harness web chat interface

dsh-message-navigator is a DSH client plugin developed by TableRogue, designed to optimize the long-conversation experience in web chat interfaces. The plugin adds a vertical message index on the right side of the interface, mapping user inputs to clickable Markers. Users can smoothly navigate to specific messages by clicking the Marker, and the Marker highlights the current reading position in real-time during scrolling. Its core features include: achieving scroll synchronization without re-rendering through pure arithmetic calculations; and stably identifying messages via `data-chat-flow-key` to ensure the index is automatically rebuilt and positioning remains accurate during message additions, deletions, or session switching. Markers are not generated during AI streaming output; they are only added to the index after the message is stabilized. Additionally, the plugin supports hover previews, features a responsive design, and automatically hides the sidebar in narrow windows.