feat(dock): add visual drop target overlay

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

@@ -15,11 +15,15 @@ namespace Ghost.Editor.View.Controls;
 public sealed partial class DockLayout : Control
 {
     private const string PART_ROOT_GRID = "PART_RootGrid";
+    private const string PART_DROP_TARGET_OVERLAY = "PART_DropTargetOverlay";
     private const double MIN_PANE_SIZE = 100;
     private const double SPLITTER_THICKNESS = 4;
 
+    private Border? _dropTargetOverlay;
     private readonly HashSet<DockNode> _subscribedNodes = new();
 
+    public enum DockPosition { Center, Top, Bottom, Left, Right, None }
+
     public DockLayout()
     {
         DefaultStyleKey = typeof(DockLayout);
@@ -287,6 +291,7 @@ public sealed partial class DockLayout : Control
     protected override void OnApplyTemplate()
     {
         base.OnApplyTemplate();
+        _dropTargetOverlay = GetTemplateChild(PART_DROP_TARGET_OVERLAY) as Border;
         RenderTree();
     }
 }

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

@@ -7,7 +7,15 @@
         <Setter Property="Template">
             <Setter.Value>
                 <ControlTemplate TargetType="local:DockLayout">
+                    <Grid>
                         <Grid x:Name="PART_RootGrid" Background="{ThemeResource ApplicationPageBackgroundThemeBrush}" />
+                        <Border x:Name="PART_DropTargetOverlay" 
+                                Background="#660078D4" 
+                                BorderBrush="#FF0078D4" 
+                                BorderThickness="2" 
+                                Visibility="Collapsed" 
+                                IsHitTestVisible="False" />
+                    </Grid>
                 </ControlTemplate>
             </Setter.Value>
         </Setter>