dsh-usage-calendar: DeepSeek Account Balance and Actual Billing Calendar

The dsh-usage-calendar plugin provides a usage tracking solution for DeepSeek, which lacks an official usage API. It samples balance decreases to calculate actual charges, aggregates token usage, cache hit rates, and pricing estimates across all local sessions, and offers a calendar view. The plugin runs entirely locally without uploading data, and supports manual balance refreshes and historical data backfilling, helping developers reconcile costs and control spending.