[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/theme_config.h

@@ -39,6 +39,14 @@ struct PaletteConfig
     QMap<QPalette::ColorGroup, QMap<QPalette::ColorRole, QColor>> colors;
     QMap<AppColor::Role, QColor> appColors;
 
+    bool operator==(const PaletteConfig &rhs) const
+    {
+        return colors == rhs.colors && appColors == rhs.appColors;
+    }
+    bool operator!=(const PaletteConfig &rhs) const
+    {
+        return !(*this == rhs);
+    }
     bool hasPalette() const;
     QString toToml() const;