dsh-provider-billing: View the Provider Account Balance in the Model Settings Row

The dsh-provider-billing plugin is specifically designed for DeepSeek Harness (DSH) to provide intuitive monitoring of account balances across multiple model providers. It displays balance information directly within each provider row on the model settings page and offers a manual refresh function. The plugin uses a loopback-pinned RPC channel for secure communication, ensuring that API keys always remain on the host side. Only the route ID is sent, which the host then resolves and uses to call the provider's `/user/balance` endpoint. The plugin requires a newer Harness build and is currently primarily compatible with the official DeepSeek API, making it suitable for scenarios that demand efficient quota management and strong security.