migrate: Qt5/C++11 app to Qt6/C++23 with tests and modern Qt APIs

Entire-Checkpoint: c1ee98dc34a3

qt6-after/qt6-migrated-benchmark/main.cpp

@@ -0,0 +1,19 @@
+#include "MainWindow.h"
+
+#include <QApplication>
+#include <QScreen>
+
+#if __has_include(<print>)
+#include <print>
+#endif
+
+[[nodiscard]] auto main(int argc, char* argv[]) -> int
+{
+    QApplication app{argc, argv};
+
+    // Qt6 uses UTF-8 by default; no codec setup needed
+    MainWindow window;
+    window.show();
+
+    return app.exec();
+}