feat: Implement chained line drawing with Esc key exit

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

src/ViewportWidget.h

@@ -71,6 +71,7 @@ protected:
     void mousePressEvent(QMouseEvent *event) override;
     void mouseMoveEvent(QMouseEvent *event) override;
     void wheelEvent(QWheelEvent *event) override;
+    void keyPressEvent(QKeyEvent *event) override;
 
 private:
     QVector3D project(const QVector3D& worldCoord, const QMatrix4x4& modelView, const QMatrix4x4& projection, const QRect& viewport);