Stop the mode slot taking half of every people row

A row's prose is Flexible so it wraps rather than overflows, and a `:vbox`
holding a label and no control is prose. The people panel's mode slot is
exactly that — a 14-point box with a dim label in it — so it was handed a
flex of its own, and two flex children of one flex each halve the row
between them. The slot drew its 14 points and left the rest of its half
empty; the nick beside it got the other half, and ellipsised at three
letters in a column with room for thirteen.

`flexed` already declines to wrap a child that has said how wide it is —
Expanded hands out a tight width and beats the SizedBox underneath. The
loose Flexible is the same bargain from the other side: it takes width
away from a child that was not asking for any. So the same question is
asked before it too, and the slot goes back to being a plain child the
row measures at 14.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>