dsh-widescreen: DeepSeek Harness Widescreen Mode

`dsh-widescreen` is a plugin for DeepSeek Harness Web UI, designed to address the whitespace caused by a fixed 748px content column on wide displays. It provides a Widescreen Mode toggle in settings, allowing full-width display for the chat column, cards, and other elements, while supporting responsive reflow and persistent preferences. It requires DSH 0.1.0-rc.x and can be enabled or uninstalled using the `dsh plugin` command. The plugin includes no build steps and integrates into general settings through the client package.