/ui page: preview, code tabs and a copy button #411

The page people actually land on from search: /ui, one card per section, live preview, and the code underneath.

Needs a copy button, HTML and CSS in separate tabs, and the theme tokens listed so a pasted block picks up the host page's colours instead of fighting them.

Done when: /ui renders every section in the registry, the copy button puts working code on the clipboard, and the preview measures over 4.5:1 on both themes.