feat: Render 3D cube with camera controls (zoom, pan, rotate)

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

src/ViewportWidget.h

@@ -0,0 +1,38 @@
+#ifndef VIEWPORTWIDGET_H
+#define VIEWPORTWIDGET_H
+
+#include <QOpenGLWidget>
+#include <QOpenGLFunctions>
+#include <QMatrix4x4>
+#include <QPoint>
+
+class ViewportWidget : public QOpenGLWidget, protected QOpenGLFunctions
+{
+    Q_OBJECT
+
+public:
+    explicit ViewportWidget(QWidget *parent = nullptr);
+
+protected:
+    void initializeGL() override;
+    void paintGL() override;
+    void resizeGL(int w, int h) override;
+
+    void mousePressEvent(QMouseEvent *event) override;
+    void mouseMoveEvent(QMouseEvent *event) override;
+    void wheelEvent(QWheelEvent *event) override;
+
+private:
+    void drawCube();
+
+    QMatrix4x4 projection;
+
+    float xRot = 0;
+    float yRot = 0;
+    float zoom = -5.0f;
+    float panX = 0;
+    float panY = 0;
+    QPoint lastPos;
+};
+
+#endif // VIEWPORTWIDGET_H