dsh-theme-win98: DeepSeek Harness Windows 98 Theme Plugin

The DeepSeek Harness (DSH) Web UI plugin `dsh-theme-win98` restyles the interface into the classic Windows 98 look. Key features include a teal desktop, dark blue gradient title bars, 3D controls, and retro scrollbars. After installation, it is enabled by default and supports fine-grained control of the global theme and desktop mode via the console API or LocalStorage. Implemented entirely on the client side, it does not modify application code; however, applying the theme globally may disable rounded corners, and some styles rely on DSH’s CSS module prefixes, so compatibility should be considered.