The channel page's underline tab pair was the last hand-rolled tab idiom on a
swept surface. It now uses the shared <Tabs> (rounded-lg pill, active accent
shadow) like every other tab-set, sitting inline with the channel action
buttons on the same separator row. Drops the bespoke tabClass helper. Only the
NavSidebar icon rail (aria-current, icon-only app nav) stays a distinct idiom.
Self-E2E'd: Videos/About switch, content flips. gate green (tsc/eslint-0/
prettier). Branch not merged.