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;
     }

src/AdvancedCalculator/Views/MainView.axaml

@@ -183,7 +183,9 @@
             <TextBlock FontFamily="{StaticResource MDI}" Text="{x:Static m:IconFont.Function}" />
         </Button>
 
-                    <TextBox Grid.Column="2" Text="{Binding InputText, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" MinHeight="44"
+                    <TextBox x:Name="InputBox" Grid.Column="2" Text="{Binding InputText, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"
+                             CaretIndex="{Binding CaretIndex, Mode=TwoWay}"
+                             MinHeight="44"
                              AutomationProperties.Name="Expression input"
                              Watermark="Type an expression (e.g., 2 + 2)">
                         <InputElement.KeyBindings>

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;
         }
     }