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

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

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+using System.Windows.Input;
+using Avalonia;
+using Avalonia.Controls;
+using Avalonia.Input;
+using DodoSSH.Client.Shell.ViewModels;
+
+namespace DodoSSH.Client.App.Views;
+
+/// <summary>
+/// The v5b in-screen tab row: one pill per open terminal, and the "+" that opens another — parameterised by
+/// <see cref="TabCommand"/> so the terminal surface and the SFTP surface can each wire a click to a different
+/// meaning over the same list. See the remark at the top of SessionTabRow.axaml.
+/// </summary>
+internal sealed partial class SessionTabRow : UserControl
+{
+    /// <summary>What a left click on a tab runs, with the tab itself as the command parameter.</summary>
+    /// <remarks>
+    /// A plain <see cref="ICommand"/> rather than a bound property read off the shell, because which command
+    /// that is is the one thing this control cannot decide for itself — the terminal surface wants
+    /// <c>SelectTabCommand</c> and the SFTP surface wants <c>SelectFilesHostCommand</c>, and only the caller
+    /// in <c>MainWindow.axaml</c> knows which screen this instance is on.
+    /// </remarks>
+    internal static readonly StyledProperty<ICommand?> TabCommandProperty =
+        AvaloniaProperty.Register<SessionTabRow, ICommand?>(nameof(TabCommand));
+
+    public SessionTabRow() => InitializeComponent();
+
+    internal ICommand? TabCommand
+    {
+        get => GetValue(TabCommandProperty);
+        set => SetValue(TabCommandProperty, value);
+    }
+
+    /// <summary>
+    /// Closes a tab on a middle click. See the identical remark on the strip this control replaced,
+    /// <c>TerminalTabs.axaml.cs</c>, for why this is <c>PointerUpdateKind</c> rather than
+    /// <c>IsMiddleButtonPressed</c>, why it fires on press rather than release, and why it is wired on the
+    /// tab's own template root rather than on the row.
+    /// </summary>
+    /// <remarks>
+    /// Not parameterised like <see cref="TabCommand"/>: closing a tab ends its shell regardless of which
+    /// screen the middle click landed on, so both rows want the same answer — <c>CloseTabCommand</c>, read
+    /// directly off this control's own <see cref="StyledElement.DataContext"/>, which is the shell on both.
+    /// </remarks>
+    private void OnTabPointerPressed(object? sender, PointerPressedEventArgs e)
+    {
+        if (sender is not Visual { DataContext: TerminalTabViewModel tab }
+            || DataContext is not MainWindowViewModel shell)
+        {
+            return;
+        }
+
+        if (e.GetCurrentPoint((Visual)sender).Properties.PointerUpdateKind
+            is not PointerUpdateKind.MiddleButtonPressed)
+        {
+            return;
+        }
+
+        e.Handled = true;
+
+        shell.CloseTabCommand.Execute(tab);
+    }
+}