dsh-cost: Real-time calculation of DeepSeek conversation costs next to the DSH input box

`dsh-cost` is a native plugin for the DSH Web platform that calculates and displays DeepSeek conversation costs in real-time. Based on DeepSeek's official pricing schedule and token usage for the `deepseek-v4-flash` and `deepseek-v4-pro` models, it automatically converts costs to CNY or USD and displays a monetary badge next to the input box. The plugin supports hovering to view input/output tokens and detailed costs for flash/pro, with currency selection and language following system settings. Amounts are kept to two decimal places (displayed as `<0.01` if less than 0.01). Its core features include: pricing only for official built-in providers without estimating third-party model prices; and separating the calculation of cache hits, misses, and writes.