Fix four memory leaks found by AddressSanitizer

- StyleEditor: QGridLayout(this) pre-empted the widget's layout slot,
  causing setLayout(main_layout) to silently fail and orphan main_layout.
  Fix: use QGridLayout() without a parent so setLayout() succeeds.

- ExportDialog: ~ExportDialog() was empty, leaving ExportDiagramLine
  heap objects in diagram_lines_ unfreed. Fix: qDeleteAll(diagram_lines_).

- GenericPanel::getItemForDiagram: when called without the bool* created
  arg, it created a parentless QTreeWidgetItem that callers immediately
  discarded. Fix: return nullptr when created==nullptr and item not found
  (all callers already guard with if (item)).

- ElementScene: m_paste_area (created in initPasteArea) was temporarily
  added/removed from the scene during XML loading but never freed in the
  destructor. Fix: delete it if not currently in the scene.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

sources/editor/elementscene.cpp

@@ -107,6 +107,9 @@ ElementScene::~ElementScene()
 
 	if (m_decorator)
 		delete m_decorator;
+
+	if (m_paste_area && !m_paste_area->scene())
+		delete m_paste_area;
 }
 
 /**

sources/editor/styleeditor.cpp

@@ -386,7 +386,7 @@ StyleEditor::StyleEditor(QETElementEditor *editor, CustomElementGraphicPart *p,
 
 	outline_color->setSizeAdjustPolicy(QComboBox::AdjustToContents);
 	filling_color->setSizeAdjustPolicy(QComboBox::AdjustToContents);
-	auto grid_layout = new QGridLayout(this);
+	auto grid_layout = new QGridLayout();
 	grid_layout->addWidget(new QLabel(tr("Contour :")), 0,0, Qt::AlignRight);
 	grid_layout->addWidget(outline_color, 0, 1);
 	grid_layout->addWidget(new QLabel(tr("Remplissage :")), 1, 0, Qt::AlignRight);

sources/exportdialog.cpp

@@ -112,6 +112,7 @@ ExportDialog::ExportDialog(
 */
 ExportDialog::~ExportDialog()
 {
+	qDeleteAll(diagram_lines_);
 }
 
 /**

sources/genericpanel.cpp

@@ -321,9 +321,11 @@ QTreeWidgetItem *GenericPanel::getItemForDiagram(Diagram *diagram,
 		if (created) *created = false;
 		return(diagram_qtwi);
 	}
-	
+
+	if (!created) return(nullptr);
+
 	diagram_qtwi = makeItem(QET::Diagram);
-	if (created) *created = true;
+	*created = true;
 	return(diagram_qtwi);
 }