dsh-theme-palettes: DeepSeek Harness Color Scheme Infrastructure

DeepSeek Harness (DSH) follows a "everything is a plugin" philosophy, yet the interface theme layer is an exception: the Appearance picker (light/dark/system) is not open to third parties, and the theme registry does not accept plugin modifications. Consequently, third-party plugins previously lacked a standard entry point for applying complete color schemes to the web interface. The `dsh-theme-palettes` introduced below adopts a different approach: rather than modifying the theme registry, it overlays palettes onto the current theme via `theme.overrideTokens`. It also provides a `themePalettes` registration API, enabling any plugin to contribute its own.