[Client] Add [AppColors] application palette roles

The home-tab buttons' gradient over the static theme background was hardcoded, and accent-derived fallbacks could not be themed or edited: QPalette's role set is closed, so any application-specific color has to live in Cockatrice's own palette layer.

- Add an AppColor::Role enum (AccentStrong / AccentSoft) stored on PaletteConfig and round-tripped from palette-<scheme>.toml under a new [AppColors] section.
- Cache the applied app colors in ThemeManager and expose appColor(Role) with a palette-accent-derived fallback; emit paletteChanged() from applyStyleAndPalette so previews, scheme switches and OS dark mode repaint palette-driven widgets.
- Fill both app roles in PaletteGenerator::fromAccent and surface them as a dedicated section in the palette editor.
- Drive the home-tab buttons from appColor() whenever the background source is the theme (any theme, not just built-ins).
- Ship AccentStrong / AccentSoft values in the Fusion and Default default palettes so the static home-tab buttons keep their classic greens.

# Conflicts:
#	cockatrice/src/interface/widgets/general/home_widget.cpp

cockatrice/src/interface/theme_config.h

@@ -3,9 +3,25 @@
 
 #include <QColor>
 #include <QMap>
+#include <QObject>
 #include <QPalette>
 #include <QString>
 
+// Application-specific color roles, layered on top of the fixed QPalette role
+// set. Stored in the same palette-<scheme>.toml under an [AppColors] section
+// and editable from the palette editor, so theme authors can control colors
+// beyond what Qt's palette can express.
+namespace AppColor
+{
+Q_NAMESPACE
+enum Role
+{
+    AccentStrong,
+    AccentSoft,
+};
+Q_ENUM_NS(Role)
+} // namespace AppColor
+
 struct ThemeConfig
 {
     QString colorScheme;
@@ -21,6 +37,7 @@ struct ThemeConfig
 struct PaletteConfig
 {
     QMap<QPalette::ColorGroup, QMap<QPalette::ColorRole, QColor>> colors;
+    QMap<AppColor::Role, QColor> appColors;
 
     bool hasPalette() const;
     QString toToml() const;