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/ViewModels/MainViewModel.cs

@@ -1,6 +1,64 @@
-namespace AdvancedCalculator.ViewModels;
+using System.Collections.ObjectModel;
+using System.Threading.Tasks;
+using CommunityToolkit.Mvvm.ComponentModel;
+using CommunityToolkit.Mvvm.Input;
+using AdvancedCalculator.Models;
+using AdvancedCalculator.Services;
+
+namespace AdvancedCalculator.ViewModels;
 
 public partial class MainViewModel : ViewModelBase
 {
-    public string Greeting => "Welcome to Avalonia!";
+    private readonly ICalculatorService _calculatorService;
+
+    public MainViewModel()
+        : this(new CalculatorService())
+    {
+    }
+
+    public MainViewModel(ICalculatorService calculatorService)
+    {
+        _calculatorService = calculatorService;
+        History = new ObservableCollection<HistoryItem>();
+        Variables = new ObservableCollection<VariableItem>();
+    }
+
+    [ObservableProperty]
+    private string _inputText = string.Empty;
+
+    [ObservableProperty]
+    private bool _isFunctionsPanelOpen;
+
+    [ObservableProperty]
+    private int _selectedHistoryIndex = -1;
+
+    public ObservableCollection<HistoryItem> History { get; }
+    public ObservableCollection<VariableItem> Variables { get; }
+
+    [RelayCommand]
+    private void ToggleFunctions()
+    {
+        IsFunctionsPanelOpen = !IsFunctionsPanelOpen;
+    }
+
+    [RelayCommand(AllowConcurrentExecutions = false)]
+    private async Task Submit()
+    {
+        var text = InputText?.Trim();
+        if (string.IsNullOrEmpty(text))
+            return;
+
+        var result = await _calculatorService.InterpretAsync(text);
+
+        History.Add(new HistoryItem { Input = text, Output = result.Output });
+
+        Variables.Clear();
+        foreach (var v in result.Variables)
+        {
+            Variables.Add(v);
+        }
+
+        InputText = string.Empty;
+        SelectedHistoryIndex = History.Count - 1;
+    }
 }