dsh-conversation-jump: DSH Web Conversation Navigation Plugin

The `dsh-conversation-jump` plugin is designed specifically for DSH Web to address the hassle of scrolling through long conversations to find content. It provides floating navigation buttons for quickly jumping to the top or bottom of user messages, as well as to the previous or next node. It features intelligent pagination: when the target is outside the visible viewport, history is loaded automatically. The plugin is hidden by default and appears only when scrolling up or when clicked, targeting only user and guide entries. It relies on the host-side "Load More" feature and does not require persistent storage.