dsh-window98-theme: Classic Windows 98 Theme for DeepSeek Harness

The `dsh-window98-theme` plugin overrides design tokens to transform the DeepSeek Harness web interface into a classic Windows 98 style. It supports both light and dark modes, featuring pixelated fonts, raised buttons, checkerboard scrollbars, and a global border-radius reset without altering the core logic. Users can install it via a command and restart to apply the changes, while individual forms can also be disabled separately.