feat: Implement in-viewport sketch plane selection

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

src/ApplicationController.h

@@ -38,9 +38,11 @@ public slots:
     bool saveDocumentAs();
 
     void beginSketchCreation();
+    void onPlaneSelected(ViewportWidget::SketchPlane plane);
     void endSketch();
 
 signals:
+    void planeSelectionModeStarted();
     void sketchModeStarted(ViewportWidget::SketchPlane plane);
     void sketchModeEnded();
     void currentFileChanged(const QString& path);