feat: Add horizontal/vertical line snapping with visual indicator

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

src/ViewportWidget.h

@@ -93,6 +93,8 @@ private:
     bool m_isSnappingOrigin = false;
     bool m_isSnappingVertex = false;
     gp_Pnt m_snapVertex;
+    bool m_isSnappingHorizontal = false;
+    bool m_isSnappingVertical = false;
 
     QMap<int, QSvgRenderer*> m_toolIcons;
     QSvgRenderer* m_cursorRenderer = nullptr;