Implement migration plan: models, service, MVVM, XAML layout, icons\n\n- Add Models: HistoryItem, VariableItem, FunctionDefinitionItem, IconFont\n- Add Services: ICalculatorService, CalculatorService wrapping interpreter\n- Update MainViewModel: properties and Submit/ToggleFunctions commands\n- Rebuild Views/MainView.axaml with variables, history, input, functions panel\n- Add BoolToGridLengthConverter for panel toggle\n- Wire Material Design Icons font as Avalonia resource in App.axaml/csproj

src/AdvancedCalculator/Converters/BoolToGridLengthConverter.cs

@@ -0,0 +1,27 @@
+using System;
+using Avalonia.Data.Converters;
+using Avalonia;
+
+namespace AdvancedCalculator.Converters;
+
+public class BoolToGridLengthConverter : IValueConverter
+{
+    public static readonly BoolToGridLengthConverter Instance = new();
+
+    public object? Convert(object? value, Type targetType, object? parameter, System.Globalization.CultureInfo? culture)
+    {
+        var isOpen = value is bool b && b;
+        // Return star when true, otherwise zero pixels
+        return isOpen ? new GridLength(1, GridUnitType.Star) : new GridLength(0, GridUnitType.Pixel);
+    }
+
+    public object? ConvertBack(object? value, Type targetType, object? parameter, System.Globalization.CultureInfo? culture)
+    {
+        if (value is GridLength gl)
+        {
+            return gl.Value > 0;
+        }
+        return false;
+    }
+}
+