fix(dock): decouple DockLayout from App and fix multi-window shutdown

src/Editor/Ghost.Editor/App.xaml.cs

@@ -47,6 +47,10 @@ public partial class App : Application
             app._secondaryWindows.Add(window);
             window.Closed += (s, e) => app._secondaryWindows.Remove(window);
         }
+        else
+        {
+            throw new InvalidOperationException("App instance is not available.");
+        }
     }
 
     internal IHost Host
@@ -151,6 +155,14 @@ public partial class App : Application
     {
         try
         {
+            // Close all secondary windows when the primary window closes
+            var secondaryWindows = _secondaryWindows.ToArray();
+            foreach (var window in secondaryWindows)
+            {
+                window.Close();
+            }
+            _secondaryWindows.Clear();
+
             Host.StopAsync().GetAwaiter().GetResult();
             Host.Dispose();
 

src/Editor/Ghost.Editor/View/Controls/DockLayout.cs

@@ -295,6 +295,8 @@ public sealed partial class DockLayout : Control
         return tabView;
     }
 
+    public event EventHandler<TabTornOffEventArgs>? TabTornOff;
+
     private void TabView_TabDroppedOutside(Microsoft.UI.Xaml.Controls.TabView sender, Microsoft.UI.Xaml.Controls.TabViewTabDroppedOutsideEventArgs args)
     {
         if (_sourceNode != null && _draggedItem != null)
@@ -304,16 +306,20 @@ public sealed partial class DockLayout : Control
             {
                 DockMutationEngine.CleanupEmptyNodes(_sourceNode);
 
-                // Create new window and register it with App to prevent GC
-                var newWindow = new Ghost.Editor.View.Windows.DockWindow(_draggedItem);
-                App.AddSecondaryWindow(newWindow);
-                newWindow.Activate();
+                // Raise event to let the host handle window creation
+                TabTornOff?.Invoke(this, new TabTornOffEventArgs(_draggedItem));
             }
 
             ClearDragOperationState();
         }
     }
 
+    public class TabTornOffEventArgs : EventArgs
+    {
+        public object TabContent { get; }
+        public TabTornOffEventArgs(object tabContent) => TabContent = tabContent;
+    }
+
     private object? _draggedItem;
     private DockPanelNode? _sourceNode;
     private DockPosition _currentDropPosition = DockPosition.None;