ui themes prototype

[wwwillchen]

Summary by cubic

Added modern UI design principles and Tailwind theming guidance to support the UI themes prototype. This sets clear layout, interaction, and palette standards and requires declaring the primary colors used.

• Migration
  • Pick 2–3 primary colors plus neutrals.
  • Update Tailwind global theme variables and document the chosen primary colors.

^{Written for commit eb2a85f. Summary will update automatically on new commits.}