dsh-meter: Display DeepSeek billing status in DSH web session

`@dshworks/dsh-meter` is a plugin for the dsh web client designed to address the unintuitive billing status in DeepSeek sessions. The plugin adds a row of real-time information below the Composer, displaying the current session cost, billing tier, and the next billing tier switch time. Users can expand a detail card via Hover or Focus to view the total session cost, request count, model used, 24-hour cost breakdown, token consumption and costs for cache hits versus fresh inputs, account balance, bonus credits, price comparison across different billing tiers, and cache savings. If there are no active sessions, users can view cold-start rates, countdowns, a 7-day billing grid, and public rate cards via `Settings → Meter`. The plugin strictly follows DeepSeek's USD/CNY exchange rate.