feat: Implement circle drawing tool

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

src/SketchCircle.cpp

@@ -0,0 +1,56 @@
+#include "SketchCircle.h"
+#include <QJsonArray>
+
+namespace
+{
+    void pointToJson(const gp_Pnt& p, QJsonArray& arr)
+    {
+        arr.append(p.X());
+        arr.append(p.Y());
+        arr.append(p.Z());
+    }
+
+    gp_Pnt jsonToPoint(const QJsonArray& arr)
+    {
+        return gp_Pnt(arr[0].toDouble(), arr[1].toDouble(), arr[2].toDouble());
+    }
+}
+
+SketchCircle::SketchCircle() : m_radius(0.0)
+{
+}
+
+SketchCircle::SketchCircle(const gp_Pnt& center, double radius)
+    : m_center(center), m_radius(radius)
+{
+}
+
+SketchObject::ObjectType SketchCircle::type() const
+{
+    return ObjectType::Circle;
+}
+
+void SketchCircle::read(const QJsonObject& json)
+{
+    m_center = jsonToPoint(json["center"].toArray());
+    m_radius = json["radius"].toDouble();
+}
+
+void SketchCircle::write(QJsonObject& json) const
+{
+    QJsonArray centerArr;
+    pointToJson(m_center, centerArr);
+    json["center"] = centerArr;
+    json["radius"] = m_radius;
+    json["type"] = "Circle";
+}
+
+const gp_Pnt& SketchCircle::center() const
+{
+    return m_center;
+}
+
+double SketchCircle::radius() const
+{
+    return m_radius;
+}