Implementeer ICommand patroon met RelayCommand voor MVVM

- Voeg RelayCommand class toe voor herbruikbare ICommand implementatie
- Vervang button click event handler door AddProductCommand in MainViewModel
- Update XAML om Command binding te gebruiken in plaats van Click event
- Verwijder business logic uit code-behind (MainWindow.xaml.cs)
- Los DataContext duplicatie op (was twee keer MainViewModel instantie)

Dit maakt de applicatie beter testbaar en volgt proper MVVM principes.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Commands/RelayCommand.cs

@@ -0,0 +1,62 @@
+using System;
+using System.Windows.Input;
+
+namespace MVVM_DEMO.Commands
+{
+    /// <summary>
+    /// A command whose sole purpose is to relay its functionality to other objects by invoking delegates.
+    /// The default return value for the CanExecute method is 'true'.
+    /// </summary>
+    public class RelayCommand : ICommand
+    {
+        private readonly Action<object?> _execute;
+        private readonly Func<object?, bool>? _canExecute;
+
+        /// <summary>
+        /// Occurs when changes occur that affect whether or not the command should execute.
+        /// </summary>
+        public event EventHandler? CanExecuteChanged
+        {
+            add { CommandManager.RequerySuggested += value; }
+            remove { CommandManager.RequerySuggested -= value; }
+        }
+
+        /// <summary>
+        /// Creates a new command that can always execute.
+        /// </summary>
+        /// <param name="execute">The execution logic.</param>
+        public RelayCommand(Action<object?> execute) : this(execute, null)
+        {
+        }
+
+        /// <summary>
+        /// Creates a new command.
+        /// </summary>
+        /// <param name="execute">The execution logic.</param>
+        /// <param name="canExecute">The execution status logic.</param>
+        public RelayCommand(Action<object?> execute, Func<object?, bool>? canExecute)
+        {
+            _execute = execute ?? throw new ArgumentNullException(nameof(execute));
+            _canExecute = canExecute;
+        }
+
+        /// <summary>
+        /// Determines whether this command can execute in its current state.
+        /// </summary>
+        /// <param name="parameter">Data used by the command. If the command does not require data to be passed, this object can be set to null.</param>
+        /// <returns>true if this command can be executed; otherwise, false.</returns>
+        public bool CanExecute(object? parameter)
+        {
+            return _canExecute == null || _canExecute(parameter);
+        }
+
+        /// <summary>
+        /// Executes the command.
+        /// </summary>
+        /// <param name="parameter">Data used by the command. If the command does not require data to be passed, this object can be set to null.</param>
+        public void Execute(object? parameter)
+        {
+            _execute(parameter);
+        }
+    }
+}