feat: Implement sketch object base class and line geometry

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

src/SketchObject.h

@@ -0,0 +1,22 @@
+#ifndef SKETCHOBJECT_H
+#define SKETCHOBJECT_H
+
+#include <QJsonObject>
+
+class SketchObject
+{
+public:
+    enum class ObjectType {
+        Line
+    };
+
+    SketchObject() = default;
+    virtual ~SketchObject() = default;
+
+    virtual ObjectType type() const = 0;
+
+    virtual void read(const QJsonObject& json) = 0;
+    virtual void write(QJsonObject& json) const = 0;
+};
+
+#endif // SKETCHOBJECT_H