dsh-balance: Add a floating model balance widget to the DeepSeek Harness Web GUI

dsh-balance is a DSH Web GUI client plugin designed to persistently display model account balances in the bottom-right corner of the page. It supports four service providers: DeepSeek, SiliconFlow, Moonshot, and OpenRouter, automatically identifying and invoking the corresponding balance interface based on the baseURL domain. Core features include a draggable floating card (displaying total balance, top-up/reward details), automatic refresh every 60 seconds (with a 30-second host-side cache), and a global toggle in the settings page (taking effect immediately). API keys can be provided in two ways: via the settings page or the credentials service. All balance queries are performed on the host side, ensuring keys are never sent to the browser. The plugin has zero runtime dependencies and requires no build process. API routes are protected by loopback and same-origin policies.