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/Views/MainView.axaml.cs

@@ -94,6 +94,9 @@ public partial class MainView : UserControl
             _vm?.InsertFunctionCommand.CanExecute(item) == true)
         {
             _vm.InsertFunctionCommand.Execute(item);
+            // Return focus to the input box so the caret is active
+            var input = this.FindControl<TextBox>("InputBox");
+            input?.Focus();
             e.Handled = true;
         }
     }