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

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

@@ -0,0 +1,29 @@
+using Avalonia;
+using Avalonia.Controls;
+
+namespace DodoSSH.Client.App.Views;
+
+/// <summary>
+/// The v5b session sidebar: QUICK ACCESS on both surfaces, SNIPS on the terminal's own. See the remark at
+/// the top of SessionSidebar.axaml.
+/// </summary>
+internal sealed partial class SessionSidebar : UserControl
+{
+    /// <summary>Whether the SNIPS section draws below QUICK ACCESS.</summary>
+    /// <remarks>
+    /// Set from the usage site rather than inferred from a surface flag on the shell, for the same reason
+    /// <see cref="SessionTabRow.TabCommand"/> is: which sections a particular instance of this control shows
+    /// is a fact about where it was placed in <c>MainWindow.axaml</c>, not one this control can read off its
+    /// own <c>DataContext</c>.
+    /// </remarks>
+    internal static readonly StyledProperty<bool> ShowsSnipsProperty =
+        AvaloniaProperty.Register<SessionSidebar, bool>(nameof(ShowsSnips));
+
+    public SessionSidebar() => InitializeComponent();
+
+    internal bool ShowsSnips
+    {
+        get => GetValue(ShowsSnipsProperty);
+        set => SetValue(ShowsSnipsProperty, value);
+    }
+}