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;
 }
 
 /**