Theme · Editing: segments sized by their labels, a thumb that covers its segment

A segmented control's segments now take their labels' widths first and
share the spare room, so a label ends in an ellipsis only when the whole
strip lacks room. The thumb's insets meet the strip's padding again, and
each segment is measured from its borderless wrapper, so the thumb covers
the chosen segment exactly. Tests hold Comfortable whole at 280px and the
thumb to the chosen segment's bounds.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

TASKS.md

@@ -420,3 +420,4 @@ Per-component lines live in `tasks/`. Tags there:
 | T38a | 2 | PASS | |
 | T38b | 1 | PASS | |
 | T39a | 1 | FAIL | a larger text size cut the density strip at 280px; ten palette colors had no name; the density and sliders untested |
+| T39a | 2 | FAIL | segments split the strip equally and cut a longer label; the thumb lost its padding's height |