Element editor: mirror a multi-line static text by its widest line

PartText::mirror() measured the whole text as one line, so a text of
several lines landed off by the width of the other lines (18 px for
"A\nLONGER LINE" at 20 pt).

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

sources/editor/graphicspart/parttext.cpp

@@ -89,8 +89,8 @@ void PartText::mirror(qreal axis_x) {
 	// at first: rotate the text:
 	QGraphicsObject::setRotation(QET::correctAngle((360-rotation()), true));
 	// then see, where we need to re-position depending on text, font ...
-	QFontMetrics qfm(font());
-	qreal textwidth  = qfm.horizontalAdvance(toPlainText());
+	// (the widest line, not the whole text measured as one line)
+	qreal textwidth  = document()->idealWidth() - 2 * document()->documentMargin();
 	// ... and angle!!!
 	qreal rot = qRound(QET::correctAngle(rotation(), true));
 	qreal c = qCos(qDegreesToRadians(rot));