Fix bugtracker #340: map "MS Shell Dlg" aliases on Qt 6/Windows

"MS Shell Dlg 2" is a Windows font alias, not a font, and projects and
settings saved on Windows carry it. Qt 5's GDI backend let Windows resolve
it to Tahoma; Qt 6's DirectWrite backend does not know the alias and falls
back to Arial, so those texts render heavier on screen and in exported PDFs.

Register the substitutions Windows itself uses, before any application
object exists so the headless export and scripting paths get them too.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

sources/main.cpp

@@ -33,6 +33,7 @@
 
 #include <QApplication>
 #include <QDomImplementation>
+#include <QFont>
 
 #include <QStyleFactory>
 #include <QtConcurrentRun>
@@ -107,6 +108,19 @@ int main(int argc, char **argv)
 	// from Qt 6.12 on; opt in explicitly for older Qt 5/6.
 	QDomImplementation::setInvalidDataPolicy(
 		QDomImplementation::ReturnNullNode);
+
+#ifdef Q_OS_WIN
+	// "MS Shell Dlg 2" is not a font but a Windows alias, and many projects
+	// and settings saved on Windows carry it. Qt 5's GDI font backend let
+	// Windows resolve it to Tahoma; Qt 6's DirectWrite backend does not know
+	// the alias and falls back to Arial, so those texts come out heavier on
+	// screen and in exported PDFs (bugtracker #340). Resolve both aliases
+	// the way Windows does. Done before any application object exists so
+	// that the headless export and scripting runs below get it too.
+	QFont::insertSubstitution("MS Shell Dlg 2", "Tahoma");
+	QFont::insertSubstitution("MS Shell Dlg", "Microsoft Sans Serif");
+#endif
+
 	//Creation and execution of the application
 	//HighDPI
 	qputenv("QT_ENABLE_HIGHDPI_SCALING", "1");