Fix crash in MappingListModel::clear() on empty list

The previous implementation used end()-1 on a potentially empty QList,
causing an invalid iterator and a Qt assertion failure. The iterator
was also invalidated after each erase() call, making the loop UB.

Replace with the standard Qt model reset pattern.

src/gui/MappingListModel.cpp

@@ -242,10 +242,9 @@ void MappingListModel::updateModel()
 
 void MappingListModel::clear()
 {
-  for (auto it = mappingList.end() - 1; it >= mappingList.begin(); --it) {
-    mappingList.erase(it);
-    updateModel();
-  }
+  beginResetModel();
+  mappingList.clear();
+  endResetModel();
 }
 
 QModelIndex MappingListModel::getIndexFromRow(int row)