refactor: Extract view cube rendering to dedicated class

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

src/ViewCube.h

@@ -0,0 +1,24 @@
+#ifndef VIEWCUBE_H
+#define VIEWCUBE_H
+
+#include <QOpenGLFunctions>
+#include <QMatrix4x4>
+#include <QtOpenGL/qgl.h> // For GLuint
+
+class ViewCube : protected QOpenGLFunctions
+{
+public:
+    ViewCube();
+
+    void initializeGL();
+    void paintGL(const QMatrix4x4& viewMatrix, int width, int height);
+
+private:
+    void createFaceTextures();
+    void drawViewCube();
+    void drawAxes();
+
+    GLuint faceTextures[6];
+};
+
+#endif // VIEWCUBE_H