fix: Prevent sketch overlap and improve plane selection visibility

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

src/ApplicationController.cpp

@@ -102,6 +102,9 @@ bool ApplicationController::saveDocumentAs()
 
 void ApplicationController::beginSketchCreation()
 {
+    if (m_activeSketch) {
+        return;
+    }
     emit planeSelectionModeStarted();
 }