feat: Implement middle-mouse rotation around grid intersection with visual pivot

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

src/Camera.h

@@ -42,6 +42,11 @@ public:
     void animateToPlaneView(int plane);
     void animateRestoreState();
 
+    void startRotation(const QVector3D& pivot);
+    void stopRotation();
+    bool isRotating() const { return m_isRotating; }
+    const QVector3D& rotationPivot() const { return m_rotationPivot; }
+
     float savedXRot() const { return m_savedXRot; }
     float savedYRot() const { return m_savedYRot; }
     float savedZoom() const { return m_savedZoom; }
@@ -60,6 +65,9 @@ private:
     float m_panX;
     float m_panY;
 
+    QVector3D m_rotationPivot;
+    bool m_isRotating = false;
+
     float m_savedXRot = 0;
     float m_savedYRot = 0;
     float m_savedZoom = -5.0f;