Give the application a settings area built from what really exists

src/DodoSSH.Client.App/Views/SettingsView.axaml.cs

@@ -0,0 +1,12 @@
+using Avalonia.Controls;
+
+namespace DodoSSH.Client.App.Views;
+
+/// <summary>
+/// The settings mode's whole chrome: its own titlebar, its own 340px rail, and whichever of its five pages
+/// <see cref="DodoSSH.Client.Shell.ViewModels.MainWindowViewModel.ActiveSettingsPage"/> names.
+/// </summary>
+internal sealed partial class SettingsView : UserControl
+{
+    public SettingsView() => InitializeComponent();
+}