fix: restore white menu bar background after language switch (Qt 6)

In Qt 6 the QMenuBar inherits the parent QMainWindow dark background
(#2C3233) instead of using the native platform style. Add explicit
QMenuBar rules alongside the existing MainWindow stylesheet so the
menu bar stays white with black text regardless of locale or restart.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: Ski90Moo

src/openstudio_lib/MainWindow.cpp

@@ -69,7 +69,11 @@ MainWindow::MainWindow(bool isPlugin, QWidget* parent)
 #endif
 
   setObjectName("MainWindow");
-  setStyleSheet("QWidget#MainWindow { background-color: #2C3233; }");
+  setStyleSheet("QWidget#MainWindow { background-color: #2C3233; } "
+                "QMenuBar { background-color: white; color: black; } "
+                "QMenuBar::item { background-color: transparent; } "
+                "QMenuBar::item:selected { background-color: #e0e0e0; } "
+                "QMenuBar::item:pressed { background-color: #d0d0d0; }");
 
   connect(m_verticalTabWidget, &VerticalTabWidget::tabSelected, this, &MainWindow::verticalTabSelected);
   connect(m_verticalTabWidget, &VerticalTabWidget::tabSelected, this, &MainWindow::onVerticalTabSelected);