Persist QColorDialog custom colors across application restarts

Qt's QColorDialog loads custom colors from QSettings on startup but
never writes them back, so user-defined colors in the color picker
are lost when the application exits.

Save and load the 16 custom color slots explicitly via QSettings
in QETApp's constructor (after initStyle) and destructor (before
other settings are flushed). This covers every QColorDialog usage
in the application transparently.

Also fixes a QColorDialog memory leak in DiagramView.

sources/qet.cpp

@@ -20,10 +20,13 @@
 #include "shortcutmanager.h"
 
 #include <limits>
+#include <QBuffer>
+#include <QColorDialog>
 #include <QGraphicsSceneContextMenuEvent>
 #include <QAction>
 #include <QFileInfo>
 #include <QSaveFile>
+#include <QSettings>
 #include <QTextStream>
 #include <QRegularExpression>
 #include <QActionGroup>
@@ -845,3 +848,55 @@ bool QET::writeToFile(QDomDocument &xml_doc, QFile *file, QString *error_message
 
 	return(true);
 }
+
+/**
+	@brief QET::saveCustomColors
+	Save the 16 QColorDialog custom colors to QSettings so they persist
+	across application restarts.
+*/
+void QET::saveCustomColors()
+{
+	QByteArray ba;
+	QBuffer buf(&ba);
+	buf.open(QIODevice::WriteOnly);
+	QDataStream s(&buf);
+	s.setVersion(QDataStream::Qt_6_0);
+	for (int i = 0; i < 16; i++)
+		s << QColorDialog::customColor(i);
+	QSettings settings;
+	settings.setValue(QStringLiteral("color/customColors"), ba);
+}
+
+/**
+	@brief QET::loadCustomColors
+	Load the 16 QColorDialog custom colors from QSettings into Qt's
+	internal custom color array.  A short or corrupt buffer is ignored
+	so that unread slots keep their default rather than turning black.
+*/
+void QET::loadCustomColors()
+{
+	QSettings settings;
+	QByteArray ba = settings.value(QStringLiteral("color/customColors")).toByteArray();
+
+	// Fall back to the legacy ungrouped key used by earlier versions.
+	if (ba.isEmpty())
+		ba = settings.value(QStringLiteral("customColors")).toByteArray();
+
+	if (ba.isEmpty())
+		return;
+
+	QBuffer buf(&ba);
+	buf.open(QIODevice::ReadOnly);
+	QDataStream s(&buf);
+	s.setVersion(QDataStream::Qt_6_0);
+
+	QColor colors[16];
+	for (int i = 0; i < 16; i++)
+		s >> colors[i];
+
+	if (s.status() != QDataStream::Ok)
+		return;
+
+	for (int i = 0; i < 16; i++)
+		QColorDialog::setCustomColor(i, colors[i]);
+}