DSH Plugin dsh-plugin-message-timeline-navigation: Add message timeline on the left side of the web session

dsh-plugin-message-timeline-navigation is a pure front-end auxiliary plugin developed specifically for the DSH Web Client, aiming to solve the problem of low efficiency in manually scrolling to locate messages during long sessions. The plugin does not modify the DSH core; instead, it displays a Codex-style message timeline on the left side of the conversation view via an overlay, where each tick corresponds to a user turn and its complete Agent reply. Core features include: previewing message cards and a cascade expansion effect when hovering over a tick; smoothly jumping to the corresponding message and briefly highlighting it when clicking a tick; and automatically highlighting the current reading turn's tick while scrolling to indicate progress. The plugin tracks only user messages to keep the timeline dense and predictable. Installation is recommended using a pnpm environment by executing the `dsh plugin add` command. It requires React.