fix(D.2b): Slice 2 — buttons inside a whole-window-Draggable frame get their Click

Visual gate 2 (user): the "Slots" toggle caption was visible but unclickable.

Root cause (UiRoot.OnMouseDown/OnMouseUp): a left-press on a non-drag-source
widget inside a whole-window-Draggable frame (the inventory window's IA-12
drag) set _windowDragTarget; OnMouseUp then early-returned before emitting the
Click. So the paperdoll Slots button (the first plain button inside the
draggable inventory frame) never received its click. Chat/toolbar buttons
escape this — their frames aren't whole-window-draggable.

Fix (toolkit, root cause not band-aid): add UiElement.HandlesClick (a virtual
opt-out parallel to IsDragSource); UiButton overrides it true; OnMouseDown
routes a HandlesClick press to the widget (like CapturesPointerDrag) instead of
the window-drag, so OnMouseUp emits the Click. 2 regression tests lock it
(HandlesClick widget in a Draggable frame emits Click; a plain one doesn't).

Build + full App suite green (596, +2).

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

src/AcDream.App/UI/UiButton.cs

@@ -75,6 +75,11 @@ public sealed class UiButton : UiElement
     /// procedurally, so the importer must not build the button's children as widgets.</summary>
     public override bool ConsumesDatChildren => true;
 
+    /// <summary>A button is interactive — it must receive its Click even inside a whole-window-Draggable
+    /// frame (e.g. the paperdoll "Slots" toggle in the inventory window), so it opts out of the
+    /// IA-12 whole-window-drag that would otherwise swallow the press.</summary>
+    public override bool HandlesClick => true;
+
     /// <summary>
     /// Returns the File id for the current <see cref="ActiveState"/>, falling back to
     /// the DirectState ("" key) if the named state is absent.