add command-line filename argument, so open-with works

NotePad/App.axaml.cs

@@ -15,7 +15,12 @@ public partial class App : Application
     {
         if (ApplicationLifetime is IClassicDesktopStyleApplicationLifetime desktop)
         {
-            desktop.MainWindow = new MainWindow();
+            string? fileToOpen = null;
+            if (desktop.Args?.Length > 0)
+            {
+                fileToOpen = desktop.Args[0];
+            }
+            desktop.MainWindow = new MainWindow(fileToOpen);
         }
 
         base.OnFrameworkInitializationCompleted();

NotePad/MainWindow.axaml.cs

@@ -17,7 +17,11 @@ namespace NotePad
         private bool _isModified;
         private string? _originalText;
 
-        public MainWindow()
+        public MainWindow() : this(null)
+        {
+        }
+
+        public MainWindow(string? fileToOpen)
         {
             InitializeComponent();
             UpdateTitle();
@@ -26,6 +30,30 @@ namespace NotePad
             textBox.TextChanged += OnTextChanged;
 
             Closing += OnWindowClosing;
+
+            if (!string.IsNullOrEmpty(fileToOpen) && File.Exists(fileToOpen))
+            {
+                _ = LoadFileAsync(fileToOpen);
+            }
+        }
+
+        private async Task LoadFileAsync(string filePath)
+        {
+            try
+            {
+                var text = await File.ReadAllTextAsync(filePath);
+                _currentFilePath = filePath;
+                _currentFileName = Path.GetFileName(filePath);
+
+                this.FindControl<TextBox>("EditorTextBox")!.Text = text;
+                _originalText = text;
+                _isModified = false;
+                UpdateTitle();
+            }
+            catch (Exception)
+            {
+                // Silently ignore file loading errors on startup
+            }
         }
 
         private void UpdateTitle()