feat: Add origin snap for sketch points and preview

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

src/ViewportWidget.h

@@ -90,6 +90,7 @@ private:
     bool m_isDefiningLine = false;
     gp_Pnt m_firstLinePoint;
     QPoint m_currentMousePos;
+    bool m_isSnappingOrigin = false;
 
     QMap<int, QSvgRenderer*> m_toolIcons;
     QSvgRenderer* m_cursorRenderer = nullptr;