fix(chat): the header measured contrast against the wrong colour space

EnsureContrast works in ABGR. I handed it the theme's RGBA on both arguments, so
it swapped red and blue in the foreground and in the background, measured a
contrast between two colours that were never on screen, and returned a result in
the wrong order -- which then went through RgbaToAbgr a second time.

On a violet surface with a teal accent that came out as dark bordeaux on dark
violet: the exact unreadable pairing the call was there to prevent. Reported from
a real screenshot, not from a test, because nothing here is testable without a
draw frame.

Every existing caller in the codebase passes ABGR -- SettingsPalette hands over
_palette.Abgr(...), SegmentedControl uses fields literally named LabelAbgr and
TrackAbgr. Mine were the only three that did not, and all three were written in
this cycle.

HellionChat/Ui/Components/Settings/LivePreviewPanel.cs

@@ -286,8 +286,11 @@ internal sealed class LivePreviewPanel : IDisposable
         draw.DrawTrackedText(
             new Vector2(listOrigin.X + 6f, listOrigin.Y + 4f),
             MockChannel,
-            ColourUtil.RgbaToAbgr(
-                ColourUtil.EnsureContrast(theme.Colors.Accent, theme.Colors.Surface, 4.5f)
+            // ABGR in, ABGR out -- see the note in ChannelHeader.
+            ColourUtil.EnsureContrast(
+                ColourUtil.RgbaToAbgr(theme.Colors.Accent),
+                ColourUtil.RgbaToAbgr(theme.Colors.Surface),
+                4.5f
             ),
             1.8f
         );

HellionChat/Ui/StyleEngine/Widgets/ChannelHeader.cs

@@ -69,6 +69,7 @@ internal static class ChannelHeader
 
         var theme = Plugin.Instance.ThemeRegistry.Active;
         var surface = theme.Colors.Surface;
+        var surfaceAbgr = ColourUtil.RgbaToAbgr(surface);
 
         var body = BodyFace(fonts);
         var meta = MetaFace(fonts);
@@ -163,7 +164,7 @@ internal static class ChannelHeader
 
         var dl = ImGui.GetWindowDrawList();
         var bottomRight = origin + new Vector2(width, height);
-        dl.AddRectFilled(origin, bottomRight, ColourUtil.RgbaToAbgr(surface));
+        dl.AddRectFilled(origin, bottomRight, surfaceAbgr);
         dl.AddLine(
             new Vector2(origin.X, bottomRight.Y - 1f),
             new Vector2(bottomRight.X, bottomRight.Y - 1f),
@@ -175,8 +176,14 @@ internal static class ChannelHeader
 
         if (plan.ShowName)
         {
-            var accent = ColourUtil.RgbaToAbgr(
-                ColourUtil.EnsureContrast(theme.Colors.Accent, surface, 4.5f)
+            // Converted first, then measured. EnsureContrast works in ABGR --
+            // handing it the theme's RGBA swaps red and blue on both arguments,
+            // so it measures a contrast that has nothing to do with what ends up
+            // on screen and returns a colour in the wrong order on top.
+            var accent = ColourUtil.EnsureContrast(
+                ColourUtil.RgbaToAbgr(theme.Colors.Accent),
+                surfaceAbgr,
+                4.5f
             );
             var x = origin.X + inset;
 
@@ -199,8 +206,10 @@ internal static class ChannelHeader
 
         if (plan.ShowDetail)
         {
-            var muted = ColourUtil.RgbaToAbgr(
-                ColourUtil.EnsureContrast(theme.Colors.TextMuted, surface, 4.5f)
+            var muted = ColourUtil.EnsureContrast(
+                ColourUtil.RgbaToAbgr(theme.Colors.TextMuted),
+                surfaceAbgr,
+                4.5f
             );
 
             var whereFace = detail.WhereIsTranslated ? body : meta;