left-pad with zero in case code is less than 6 digits

mainwindow.cpp

@@ -194,5 +194,5 @@ QString MainWindow::generateCode(QString key) {
   p = CppTotp::totp(qui, time(NULL), 0, 30, 6);
 
   // return only the first six digits, as a string, with a space in the middle
-  return QString::number(p).left(6).replace(QRegularExpression(R"((\d{3})(\d{3}))"), R"(\1 \2)");
+  return QString::number(p).rightJustified(6, '0', true).replace(QRegularExpression(R"((\d{3})(\d{3}))"), R"(\1 \2)");
 }