fix(editor): improve DockContainer robustness and style consistency

- Added cycle detection in AddChild to prevent tree-cycle bugs
- Added defensive null validation to AddChild and RemoveChild
- Standardized using directives and exception throwing style

src/Editor/Ghost.Editor/View/Controls/Docking/DockContainer.cs

@@ -1,3 +1,4 @@
+using System;
 using System.Collections.ObjectModel;
 
 namespace Ghost.Editor.View.Controls.Docking;
@@ -20,8 +21,21 @@ public abstract class DockContainer : DockModule
 
     public void AddChild(DockModule module)
     {
+        ArgumentNullException.ThrowIfNull(module);
+
         if (module == this)
-            throw new System.ArgumentException("Cannot add a container to itself.", nameof(module));
+            throw new ArgumentException("Cannot add a container to itself.", nameof(module));
+
+        if (module is DockContainer container)
+        {
+            var current = Owner;
+            while (current != null)
+            {
+                if (current == container)
+                    throw new ArgumentException("Cannot add a container that is an ancestor of this container.", nameof(module));
+                current = current.Owner;
+            }
+        }
 
         if (_children.Contains(module))
             return;
@@ -33,6 +47,8 @@ public abstract class DockContainer : DockModule
 
     public void RemoveChild(DockModule module)
     {
+        ArgumentNullException.ThrowIfNull(module);
+
         if (_children.Remove(module))
         {
             module.Owner = null;