feat(scene): add canvas and element context menus

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/scene/GridView.cpp

@@ -2,6 +2,8 @@
 
 #include "DiagramScene.h"
 
+#include <QContextMenuEvent>
+#include <QMenu>
 #include <QMimeData>
 #include <QMouseEvent>
 #include <QPainter>
@@ -153,6 +155,28 @@ void GridView::dropEvent(QDropEvent* event)
         event->acceptProposedAction();
 }
 
+void GridView::contextMenuEvent(QContextMenuEvent* event)
+{
+    QGraphicsItem* item = itemAt(event->pos());
+    QMenu menu(this);
+    if (item) {
+        // Right-clicking an unselected element makes it the selection.
+        if (!item->isSelected()) {
+            scene()->clearSelection();
+            item->setSelected(true);
+        }
+        menu.addAction(tr("Copy"), m_scene, &DiagramScene::copySelection);
+        menu.addAction(tr("Cut"), m_scene, &DiagramScene::cutSelection);
+        menu.addAction(tr("Delete"), m_scene, &DiagramScene::deleteSelection);
+    } else {
+        QAction* pasteAct = menu.addAction(tr("Paste"), m_scene, &DiagramScene::paste);
+        pasteAct->setEnabled(m_scene->canPaste());
+        menu.addAction(tr("Select All"), m_scene, &DiagramScene::selectAll);
+        menu.addAction(tr("Zoom to Fit"), this, &GridView::zoomToFit);
+    }
+    menu.exec(event->globalPos());
+}
+
 bool GridView::handleNodeTypeDrop(const QMimeData* mime, QPoint viewportPos)
 {
     if (!mime || !mime->hasFormat(QLatin1String(kNodeTypeMime)))

src/scene/GridView.h

@@ -41,6 +41,7 @@ protected:
     void dragEnterEvent(QDragEnterEvent* event) override;
     void dragMoveEvent(QDragMoveEvent* event) override;
     void dropEvent(QDropEvent* event) override;
+    void contextMenuEvent(QContextMenuEvent* event) override;
 
 private:
     void applyZoom(double factor);