Editorial horizontal tab row used to pick a tutor mode (Socratic /
Confused Student) per tutor.html:21-30. The active tab gets a 2 px
ochre underline and the row sits on a hairline bottom border. Each tab
takes an equal share of the row's width via Expanded, with the 28 px
inter-tab gap living as right padding on every tab except the last —
this keeps the layout safe at iPhone-class widths where intrinsic-width
tabs overflow the available space.