feat: place caret inside inserted function parentheses\n\n- Add CaretIndex to MainViewModel and bind TextBox.CaretIndex\n- After function insertion, set caret to inside parentheses\n- Return focus to input box for immediate typing

src/AdvancedCalculator/ViewModels/MainViewModel.cs

@@ -35,6 +35,10 @@ public partial class MainViewModel : ViewModelBase
     [ObservableProperty]
     private int _selectedHistoryIndex = -1;
 
+    // Caret position in the input TextBox
+    [ObservableProperty]
+    private int _caretIndex;
+
     public ObservableCollection<HistoryItem> History { get; }
     public ObservableCollection<VariableItem> Variables { get; }
 
@@ -61,6 +65,11 @@ public partial class MainViewModel : ViewModelBase
         if (string.IsNullOrWhiteSpace(functionDefintionItem.FunctionName))
             return;
         InsertToken(functionDefintionItem.FunctionName + "()");
+        // Place caret inside the parentheses
+        if (!string.IsNullOrEmpty(InputText))
+        {
+            CaretIndex = Math.Max(0, InputText.Length - 1);
+        }
         // Close the functions flyout after insertion for smoother UX
         IsFunctionsPanelOpen = false;
     }