Theme · Platform: a title bar in three parts, buttons at the window's rem

A title bar lays out a start, a title and an end. In the centered styles
the start and end grow alike from nothing, their padding inside them, so
a short title sits at the bar's center and a long one gives way before
whatever stands at either end; on Windows the title, or a spacer when
there is none, takes the free width before the buttons. Caption buttons
take the bar's height at the window's own rem. Tests keep an untitled
Windows bar's buttons at its end, a long Linux title before the round
buttons, a short title centered, and the buttons inside a bar at rem 8.

TASKS.md

@@ -425,3 +425,4 @@ Per-component lines live in `tasks/`. Tags there:
 | T39b | 1 | FAIL | a long theme name pushed its badge past 280px; four-digit colors refused the theme |
 | T39b | 2 | PASS | |
 | T39c | 1 | FAIL | Windows caption buttons took their row's height; a long Windows title pushed the buttons past the bar |
+| T39c | 2 | FAIL | an untitled Windows bar lost its spacer; a centered title ran under the Linux buttons; caption buttons ignored the window's rem |