suanzhang-dsh: Display DeepSeek Harness balance and step costs in the interface

suanzhang-dsh is a local billing plugin designed for DeepSeek Harness (dsh), aimed at addressing the need for users to intuitively view API call costs and balances. This plugin does not invoke the models; it calculates costs solely by reading local session logs and persistently displays the current balance and daily spending in the sidebar of the web interface. Its core functions include providing a detailed cost breakdown by step and supporting visualized summaries and predictions of costs by model, tool, day, and session. It automatically syncs official pricing and falls back to built-in pricing upon failure. It supports DeepSeek V4-Flash/Pro (including peak-valley pricing) and deepseek-chat/reasoner, while marking other models as unbilled. Users can click on a cost row to navigate to the corresponding trace step. Installation requires Node.js v22+, and is supported from GitHub.