Add macOS support and CI

src/macos_titlebar.mm

@@ -0,0 +1,43 @@
+#include "macos_titlebar.h"
+#include "themes/theme.h"
+
+#import <Cocoa/Cocoa.h>
+#include <QColor>
+#include <QWidget>
+
+namespace rcx {
+
+static NSColor* toNSColor(const QColor& color) {
+    return [NSColor colorWithCalibratedRed:color.redF()
+                                     green:color.greenF()
+                                      blue:color.blueF()
+                                     alpha:color.alphaF()];
+}
+
+void applyMacTitleBarTheme(QWidget* window, const Theme& theme) {
+    if (!window) return;
+
+    // Ensure native window is created.
+    window->winId();
+
+    auto* nsView = reinterpret_cast<NSView*>(window->winId());
+    if (!nsView) return;
+
+    NSWindow* nsWindow = [nsView window];
+    if (!nsWindow) return;
+
+    // Keep native traffic lights while tinting the title bar to the theme.
+    // Match the title text contrast by selecting the appropriate system appearance.
+    const qreal luminance =
+        0.2126 * theme.background.redF() +
+        0.7152 * theme.background.greenF() +
+        0.0722 * theme.background.blueF();
+    const bool isLight = luminance >= 0.5;
+    [nsWindow setAppearance:[NSAppearance appearanceNamed:
+        (isLight ? NSAppearanceNameAqua : NSAppearanceNameDarkAqua)]];
+    [nsWindow setTitlebarAppearsTransparent:YES];
+    [nsWindow setTitleVisibility:NSWindowTitleVisible];
+    [nsWindow setBackgroundColor:toNSColor(theme.background)];
+}
+
+} // namespace rcx