refactor: Extract view cube rendering to dedicated class

Co-authored-by: aider (gemini/gemini-2.5-pro) <aider@aider.chat>

src/ViewportWidget.h

@@ -6,12 +6,15 @@
 #include <QMatrix4x4>
 #include <QPoint>
 
+class ViewCube;
+
 class ViewportWidget : public QOpenGLWidget, protected QOpenGLFunctions
 {
     Q_OBJECT
 
 public:
     explicit ViewportWidget(QWidget *parent = nullptr);
+    ~ViewportWidget();
 
 protected:
     void initializeGL() override;
@@ -24,12 +27,9 @@ protected:
 
 private:
     void drawCube();
-    void drawViewCube();
-    void drawAxes();
-    void createFaceTextures();
 
     QMatrix4x4 projection;
-    GLuint faceTextures[6];
+    ViewCube* m_viewCube;
 
     float xRot = 0;
     float yRot = 0;