dsh-user-message-navigator: Add user message navigation sidebar for DeepSeek Harness Web

dsh-user-message-navigator is a DSH plugin for DeepSeek Harness Web (version 0.1.1, MIT license) designed to improve the retrieval efficiency of user messages in long conversations. It provides a Codex-style sidebar that centrally lists direct user prompts and accepted guided messages in chronological order. It supports local search, click-to-jump, momentary highlighting, and pagination loading of history, while following the current language and theme. The plugin only indexes currently visible conversation nodes, excluding generated context and compressed nodes. All operations are performed locally in the browser without network requests, ensuring privacy. It is suitable for scenarios involving frequent message review and multi-turn dialogues. Installation requires DeepSeek Harness Web 0.1.1-rc.2 or higher and Node.js ^22.19.