feat(chrome/lab): add about — page-foot framing block

Adds chrome/lab/about.css to cover the .about / .about-card / .about-label / .about-lead / .about-thanks pattern used by lab evidence sites (currently rootless; the other 6 lab targets all need the same).

Discovered while bringing lab/rootless onto the published chrome — the existing site renders the about section unstyled because no equivalent component existed.

Markup contract documented at the top of the file. Two-column card that collapses to single-column under 720px. Tokens-only.