dsh-better-stats: Add a real-time cost and status bar to DSH Web UI

# Introduction When running long tasks with DSH (DeepSeek Harness), keeping track of costs is the most difficult part: DeepSeek-V4-Flash and DeepSeek-V4-Pro have different pricing, and official RMB rates vary by peak and off-peak hours. Input is charged in buckets based on cache hits or misses, and the usage of parallel sub-sessions within an agent team must be aggregated. It is hard to estimate how much the current turn has cost halfway through the streaming output, and manually reconciling costs using token details afterward is also a hassle. The philosophy behind DSH is "everything is a plugin," and information display in the Web UI can similarly be handled by plugins. dsh-better-stats does exactly this: it places a status bar directly below the Composer,