fix(D.5.1): draw window-frame border over content (OnDrawAfterChildren)

UiNineSlicePanel drew its full chrome in OnDraw, before children, so content painted OVER the frame. The toolbar's row-2 right cap (0x100006C0, W=8) extends 2px past the 300px content and was poking over the frame's bottom-right border (the 'missing frame' the user circled). Split the panel: center fill stays in OnDraw (background, under content); the bevel border + grip move to a new UiElement.OnDrawAfterChildren hook (foreground, over content edges) so the frame is the outermost layer. Chat is unaffected (its content is inset 5px, so the border never overlaps it).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/AcDream.App/UI/UiElement.cs

@@ -167,6 +167,15 @@ public abstract class UiElement
     /// </summary>
     protected virtual void OnDraw(UiRenderContext ctx) { }
 
+    /// <summary>
+    /// Draw AFTER this element's own children, but still within this element's
+    /// transform/alpha (NOT a global pass like <see cref="OnDrawOverlay"/>). Use for a
+    /// window FRAME border, which must be the outermost layer drawn OVER its content's
+    /// edges (so content can't poke through the frame), while the frame's center fill
+    /// stays a background in <see cref="OnDraw"/>. Default: nothing.
+    /// </summary>
+    protected virtual void OnDrawAfterChildren(UiRenderContext ctx) { }
+
     /// <summary>
     /// Draw content that must sit ON TOP of the ENTIRE UI, regardless of this
     /// element's position in the tree — open menus, dropdowns, tooltips. Called in
@@ -228,6 +237,10 @@ public abstract class UiElement
                 for (int i = 0; i < ordered.Length; i++)
                     ordered[i].DrawSelfAndChildren(ctx);
             }
+
+            // Foreground pass for this element (e.g. a window frame's border drawn
+            // OVER its content's edges). Default no-op for ordinary elements.
+            OnDrawAfterChildren(ctx);
         }
         finally
         {