dsh-conversation-flat: DeepSeek Harness Layout Optimization and Profile Plugin

The `dsh-conversation-flat` plugin changes the default centered narrow-bubble layout of DeepSeek Harness (DSH) to a full-width document flow layout and adds profile features. User messages become full-width light-colored bars, assistant replies display a “DeepSeek” label, and Markdown tables automatically stretch to fill column widths. The plugin supports customizing width, colors, and labels in `bundle/client.js`. Settings are saved on the server, while avatars and names are dynamically rebuilt during streaming output. It depends on `schemastery` and is compatible with dsh 0.1.2-alpha.5.