deepseek-files-panel: Manage DeepSeek Files API uploads on the DSH Web Interface

**Introduction** When running agents via the DeepSeek Harness (DSH) web interface, the official adapter supports the DeepSeek Files API starting from version 0.1.1-rc.2+. Images are uploaded via the Files API and reuse `file_id`s. These files are stored on the DeepSeek cloud and expire after 7 days by default. Previously, to check which files are in the account and delete unused ones, users had to manually call the `https://api.deepseek.com/files` endpoint or write a temporary script. The `deepseek-files-panel` integrates these operations into the DSH web interface: a collapsible panel on the right side that lists, copies, and cleans up files uploaded via...