Show scroll bars at full width, drop the log status bar

- Turn off scroll bar auto-hide for ScrollViewer and DataGrid.
- Make the splitter between the two log grids 5 px, in the grid line color.
- Remove the log window status bar. The stats are in the entry window.
- Show a refused log edit in an alert dialog.
- Keep the log grid's rows in an ObservableCollection, so a new contact
  does not rebuild the list and move the view.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

src/Nonemm.App/Dialogs/AlertDialog.axaml.cs

@@ -0,0 +1,16 @@
+using Avalonia.Controls;
+using Avalonia.Interactivity;
+
+namespace Nonemm.App.Dialogs;
+
+public sealed partial class AlertDialog : Window
+{
+    public AlertDialog(string title, string message)
+    {
+        InitializeComponent();
+        Title = title;
+        MessageText.Text = message;
+    }
+
+    private void OnClose(object? sender, RoutedEventArgs e) => Close();
+}