Baseline: WPF/Avalonia/Cli/Core + plans (BROWSER_FEATURES_PLAN v2, MOBILE_PLAN v1)

VideoDownloader.Avalonia/RelayCommand.cs

@@ -0,0 +1,16 @@
+using System.Windows.Input;
+
+namespace VideoDownloader.Avalonia;
+
+/// <summary>간단한 ICommand 구현(CommunityToolkit 없이 MVVM 커맨드 바인딩용).</summary>
+public sealed class RelayCommand : ICommand
+{
+    private readonly Action<object?> _execute;
+    private readonly Func<object?, bool>? _canExecute;
+    public RelayCommand(Action<object?> execute, Func<object?, bool>? canExecute = null)
+    { _execute = execute; _canExecute = canExecute; }
+    public bool CanExecute(object? parameter) => _canExecute?.Invoke(parameter) ?? true;
+    public void Execute(object? parameter) => _execute(parameter);
+    public event EventHandler? CanExecuteChanged;
+    public void RaiseCanExecuteChanged() => CanExecuteChanged?.Invoke(this, EventArgs.Empty);
+}