Move functions list to flyout and auto-close after insert

src/AdvancedCalculator/ViewModels/MainViewModel.cs

@@ -56,6 +56,8 @@ public partial class MainViewModel : ViewModelBase
         if (string.IsNullOrWhiteSpace(functionName))
             return;
         InsertToken(functionName + "()");
+        // Close the functions flyout after insertion for smoother UX
+        IsFunctionsPanelOpen = false;
     }
 
     private void InsertToken(string token)