fix: Move line tool specific logic from ViewportWidget to LineTool

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

src/ViewportWidget.h

@@ -52,6 +52,8 @@ public:
     const gp_Pnt& snapVertex() const { return m_snapVertex; }
     bool isSnappingHorizontal() const { return m_isSnappingHorizontal; }
     bool isSnappingVertical() const { return m_isSnappingVertical; }
+    void setSnappingHorizontal(bool snapping);
+    void setSnappingVertical(bool snapping);
 
     void addLine(const gp_Pnt& start, const gp_Pnt& end);
     void deactivateActiveTool();