Give the window its v5b chrome and each session surface its own shell

tests/DodoSSH.Client.App.Tests/VaultVisibilityTests.cs

@@ -639,7 +639,13 @@ public sealed class VaultVisibilityTests : IAsyncLifetime
         vault.IsEditing.ShouldBeFalse(vault.Status);
     }
 
-    /// <summary>Switches a vault off through the menu, as the tab strip does.</summary>
+    /// <summary>
+    /// Switches a vault off through <see cref="MainWindowViewModel.ToggleVaultCommand"/> — the command
+    /// behind a vault row in the rail's own user popover now, not the tab strip's old vault menu, which is
+    /// gone; see <c>NavRail.axaml</c>. This project is deliberately Avalonia-free, so what this file proves
+    /// is the command's own effect; that the popover's row is wired to call it is
+    /// <c>DodoSSH.Client.App.Layout.Tests.NavRailTests</c>' business.
+    /// </summary>
     private async Task HideAsync(Guid vaultId)
     {
         var toggle = shell.VaultToggles.Single(row => row.VaultId == vaultId);