refactor: Introduce ApplicationController to centralize logic

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

src/ViewportWidget.h

@@ -34,10 +34,12 @@ public:
     explicit ViewportWidget(QWidget *parent = nullptr);
     ~ViewportWidget();
 
-    void startSketch(SketchPlane plane);
-    void saveSketch();
     void setDocument(Document* document);
 
+public slots:
+    void onSketchModeStarted(SketchPlane plane);
+    void onSketchModeEnded();
+
     float xRotation() const;
     void setXRotation(float angle);