Give the application a settings area built from what really exists

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

@@ -0,0 +1,9 @@
+using Avalonia.Controls;
+
+namespace DodoSSH.Client.App.Views;
+
+/// <summary>Settings mode's own 340px rail — General/Vaults/Account, Security/Preferences, and Logout.</summary>
+internal sealed partial class SettingsNav : UserControl
+{
+    public SettingsNav() => InitializeComponent();
+}