[AppColors] Address review comments

- PaletteEditorDialog::onSave(): compare whole PaletteConfig (colors and
  appColors) so a change to only AccentStrong/AccentSoft writes the file;
  add PaletteConfig::operator==.
- appColor(): derive both roles from QPalette::Highlight unconditionally.
  The Fusion palettes pin Accent to near-Window values, and QPalette::Accent
  only exists on Qt 6.6+, so keying on it made identical themes render very
  differently across Qt versions.
- themeChangedSlot(): merge the theme default's [AppColors] into a custom
  palette that predates the section instead of all-or-nothing per file;
  hasPalette() now counts an appColors-only file as a palette.
- Add Default/palette-default-light.toml so the Default theme's Light scheme
  keeps the classic greens instead of falling back to the OS accent.
- home_widget: restore the isBuiltInTheme() half of the Automatic condition;
  non-built-in themes extract button colors from their own background art.
- palette_grid_widget: use appEnum.value(i) for the role cast (3 sites),
  append appHeader to headerLabels, fix the 'Lighted' typo.

cockatrice/src/interface/widgets/general/home_widget.cpp

@@ -124,10 +124,13 @@ QPair<QColor, QColor> HomeWidget::determineButtonColor() const
 
     switch (colorSource) {
         case HomeTabButtonColor::Automatic: {
-            if (usesThemeBackground()) {
-                // Static theme background: follow the theme's accent colors.
+            if (usesThemeBackground() && themeManager->isBuiltInTheme()) {
+                // Built-in themes paint a static theme background; follow the
+                // theme's identity accent colors rather than sampling the image.
                 return paletteDerivedButtonColors();
             } else {
+                // Non-built-in themes may ship their own background art, so
+                // extract the button colors from the image actually painted.
                 return extractDominantColors(background);
             }
         }