Use explicit cursor index type for Qt 5 and Qt 6

tools/chatgui/src/messagedelegate.cpp

@@ -1378,7 +1378,7 @@ void MessageDelegate::paint(QPainter *painter, const QStyleOptionViewItem &optio
         layoutInviteText(layout, L.textRect.width());
         QVector<QTextLayout::FormatRange> selections;
         if (singleTextSel && cv) {
-            auto cursorAt = [&](const QPoint& point) {
+            auto cursorAt = [&](const QPoint& point) -> int {
                 for (int i = 0; i < layout.lineCount(); ++i) {
                     const auto line = layout.lineAt(i);
                     if (point.y() < line.y() + line.height()) return line.xToCursor(point.x());