fix: Add and emit toolDeactivated signal in ViewportWidget

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

src/ViewportWidget.h

@@ -47,6 +47,7 @@ public slots:
 signals:
     void lineAdded(const gp_Pnt& start, const gp_Pnt& end);
     void planeSelected(SketchPlane plane);
+    void toolDeactivated();
 
 protected:
     void initializeGL() override;