feat: Implement sketch object base class and line geometry

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

src/SketchLine.h

@@ -0,0 +1,26 @@
+#ifndef SKETCHLINE_H
+#define SKETCHLINE_H
+
+#include "SketchObject.h"
+#include <gp_Pnt.hxx>
+
+class SketchLine : public SketchObject
+{
+public:
+    SketchLine();
+    SketchLine(const gp_Pnt& start, const gp_Pnt& end);
+
+    ObjectType type() const override;
+
+    void read(const QJsonObject& json) override;
+    void write(QJsonObject& json) const override;
+
+    const gp_Pnt& startPoint() const;
+    const gp_Pnt& endPoint() const;
+
+private:
+    gp_Pnt m_start;
+    gp_Pnt m_end;
+};
+
+#endif // SKETCHLINE_H