Add collapsed group child count display

Show "(N)" in the value area when a property group is collapsed,
indicating how many properties are inside. Also restore original
expand/unexpand icon logic.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

PropertyEditor/source/src/QQuickDetailsViewLayoutBuilder.cpp

@@ -35,8 +35,8 @@ QPair<QQuickItem*, QQuickItem*> QQuickDetailsViewRowBuilder::makeNameValueSlot()
                 x: padding + (detailsDelegate.depth * detailsDelegate.indent)
                 anchors.verticalCenter: parent.verticalCenter
                 mipmap: true 
-                source: detailsDelegate.expanded 
+                source: detailsDelegate.expanded
-                    ? "qrc:/resources/Icon/expand.png" 
+                    ? "qrc:/resources/Icon/expand.png"
                     : "qrc:/resources/Icon/unexpand.png"
 
                 width: 12

PropertyEditor/source/src/QQuickDetailsViewRow.cpp

@@ -165,5 +165,27 @@ void QDetailsViewRow_Group::setupItem(QQuickItem* inParent)
 		}
 	)");
 	groupLabel->setProperty("lableText", mName);
+
+	int childCount = mChildren.size();
+	QQuickItem* countLabel = builder.setupItem(slotPair.second, R"qml(
+		import QtQuick;
+		import QtQuick.Controls;
+		import ColorPalette;
+		Item{
+			property int groupChildCount: 0
+			implicitHeight: 25
+			width: parent.width
+			anchors.verticalCenter: parent.verticalCenter
+			visible: !detailsDelegate.expanded
+			Text {
+				anchors.right: parent.right
+				anchors.verticalCenter: parent.verticalCenter
+				text: "(" + groupChildCount + ")"
+				color: ColorPalette.theme.textPrimary
+			}
+		}
+	)qml");
+	countLabel->setProperty("groupChildCount", childCount);
+
 	builder.setHeightProxy(groupLabel);
 }