feat: Add vertex snapping and draw line endpoints

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

src/ViewportWidget.h

@@ -91,6 +91,8 @@ private:
     gp_Pnt m_firstLinePoint;
     QPoint m_currentMousePos;
     bool m_isSnappingOrigin = false;
+    bool m_isSnappingVertex = false;
+    gp_Pnt m_snapVertex;
 
     QMap<int, QSvgRenderer*> m_toolIcons;
     QSvgRenderer* m_cursorRenderer = nullptr;