dsh-billing: Displaying Account Balance and Session Costs in DeepSeek Harness

`dsh-billing` is a DeepSeek Harness plugin designed to display the DeepSeek account balance and current session costs (in CNY) in real-time within the Web UI. It addresses the inconvenience of manually checking bills during conversations, offering three query interfaces: a persistent dual-capsule display at the session header, slash commands (`/balance`, `/cost`), and a model tool (`deepseek_billing`). Key features include instant data refresh upon clicking the capsule, hover-to-view details for recharges, bonuses, USD conversions, and token/cost breakdowns by model. The plugin utilizes event-driven refresh with zero requests when idle and supports automatic synchronization of the latest official pricing (including peak/off-peak rates) every 12 hours. Users can configure pricing via `config.pricing` and `config.`