Ensure Mixxx uses "windowsvista" Qt style on Windows, as our skinning…

… looks inconsistent with Qt's "windows" or "windows11" styles

Add logging of the selected style on all platforms

src/mixxxapplication.cpp

@@ -98,6 +98,19 @@ MixxxApplication::MixxxApplication(int& argc, char** argv)
           m_isDeveloper(CmdlineArgs::Instance().getDeveloper()) {
     registerMetaTypes();
 
+#if defined(Q_OS_WIN)
+    // The Mixxx style is based on Qt's WindowsVista style
+    QApplication::setStyle("windowsvista");
+#endif
+
+    qInfo() << "Selected Qt style:" << QApplication::style()->objectName();
+
+#if defined(Q_OS_WIN)
+    VERIFY_OR_DEBUG_ASSERT(QApplication::style()->objectName() == "windowsvista") {
+        qWarning() << "Failed to set Qt style to windowsvista";
+    }
+#endif
+
     // Increase the size of the global thread pool to at least
     // 4 threads, even if less cores are available. These threads
     // will be used for loading external libraries and other tasks.