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

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

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+using System.Windows.Input;
+using Avalonia;
+using Avalonia.Controls;
+
+namespace DodoSSH.Client.App.Views;
+
+/// <summary>
+/// The v5b session shell's host header: the address, and a ghost button that crosses to the other surface.
+/// See the remark at the top of SessionHeader.axaml.
+/// </summary>
+internal sealed partial class SessionHeader : UserControl
+{
+    /// <summary>What the cross-surface ghost button says — "Open SFTP" or "Open terminal".</summary>
+    internal static readonly StyledProperty<string?> OpenLabelProperty =
+        AvaloniaProperty.Register<SessionHeader, string?>(nameof(OpenLabel));
+
+    /// <summary>What the cross-surface ghost button runs.</summary>
+    /// <remarks>
+    /// The terminal usage binds <c>SelectFilesHostCommand</c> with the selected tab as its parameter; the
+    /// SFTP usage binds <c>OpenTerminalForFilesHostCommand</c>, which needs none — see the remark on both in
+    /// <c>MainWindowViewModel</c> for why the two directions are not symmetrical.
+    /// </remarks>
+    internal static readonly StyledProperty<ICommand?> OpenCommandProperty =
+        AvaloniaProperty.Register<SessionHeader, ICommand?>(nameof(OpenCommand));
+
+    internal static readonly StyledProperty<object?> OpenCommandParameterProperty =
+        AvaloniaProperty.Register<SessionHeader, object?>(nameof(OpenCommandParameter));
+
+    /// <summary>What the header says instead of an address, while no session/host context is active.</summary>
+    internal static readonly StyledProperty<string?> EmptyTextProperty =
+        AvaloniaProperty.Register<SessionHeader, string?>(nameof(EmptyText));
+
+    public SessionHeader() => InitializeComponent();
+
+    internal string? OpenLabel
+    {
+        get => GetValue(OpenLabelProperty);
+        set => SetValue(OpenLabelProperty, value);
+    }
+
+    internal ICommand? OpenCommand
+    {
+        get => GetValue(OpenCommandProperty);
+        set => SetValue(OpenCommandProperty, value);
+    }
+
+    internal object? OpenCommandParameter
+    {
+        get => GetValue(OpenCommandParameterProperty);
+        set => SetValue(OpenCommandParameterProperty, value);
+    }
+
+    internal string? EmptyText
+    {
+        get => GetValue(EmptyTextProperty);
+        set => SetValue(EmptyTextProperty, value);
+    }
+}