refactor: Implement Rectangle tool with snapping and dimensions

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

src/ViewportWidget.h

@@ -46,6 +46,7 @@ public slots:
 
 signals:
     void lineAdded(const gp_Pnt& start, const gp_Pnt& end);
+    void rectangleAdded(const gp_Pnt& corner1, const gp_Pnt& corner2);
     void planeSelected(SketchPlane plane);
     void toolDeactivated();
 
@@ -91,6 +92,8 @@ private:
     int m_activeTool = 0;
     bool m_isDefiningLine = false;
     gp_Pnt m_firstLinePoint;
+    bool m_isDefiningRectangle = false;
+    gp_Pnt m_firstRectanglePoint;
     QPoint m_currentMousePos;
     bool m_isSnappingOrigin = false;
     bool m_isSnappingVertex = false;