fix(style): give the settings pane the contrast it was missing

The converted tab had the right structure and no legibility. Five separate
causes, all of them contrast rather than colour choice.

The content pane never set ChildBg, so it inherited whatever showed through the
window, and the window is translucent by default. Settings text was sitting on
moving scenery. It gets the surface tone now; the window's own opacity still
applies on top, so the glass look survives.

Section headings had no ground of their own and floated between the rows at
roughly their weight, which meant the tab had lost its grouping entirely. They
get the raised surface, and their accent bar now runs the full height instead of
stopping at the title line.

An off toggle was filled with the same surface tone as the row behind it and
read as empty space. It gets an outline that fades out as it turns on, where the
filled track carries the shape by itself.

Two colours could not be picked statically at all, because themes here range
from near-black to pastel: the label on a selected segment, and the knob on the
track. Both now derive from the luminance of what they sit on, so neither can
end up light-on-light. That is what ColourUtil.OnColour is for.

Descriptions drop from TextMuted to TextFaint. Level with the label they made
each row read as two settings rather than one with an explanation.

HellionChat/Ui/Windows/WidgetGalleryWindow.cs

@@ -160,6 +160,7 @@ internal sealed class WidgetGalleryWindow : Window
             TrackOffAbgr = _palette.Abgr(Token.SurfaceRaised, c),
             TrackOnAbgr = _palette.Abgr(Token.AccentPrimary, c),
             KnobAbgr = _palette.Abgr(Token.Text, c),
+            BorderAbgr = _palette.Abgr(Token.Border, c),
         };
 
         var size = ToggleSwitch.CalcSize();