feat(D.2b): wire UiHost input + moveable windows (UiRoot window-drag + WantCapture gate)

- UiElement: add Draggable flag; left-drag on a draggable element repositions
  it as a floating window instead of starting a drag-drop sequence.
- UiRoot: add WantsMouse/WantsKeyboard properties (mirrors ImGui's WantCaptureMouse
  pattern); add FindDraggable helper; inject _windowDragTarget state machine into
  OnMouseDown/OnMouseMove/OnMouseUp so draggable windows track the pointer offset.
- UiNineSlicePanel: set Draggable=true so retail window frames are movable by default.
- GameWindow: OR _uiHost?.Root.WantsMouse|WantsKeyboard into the SilkMouseSource
  wantCaptureMouse/wantCaptureKeyboard delegates and the direct MouseMove gate so
  game actions (movement, world-pick) are suppressed while the pointer is over a
  retail window — no double-handling with the InputDispatcher.
- GameWindow: wire all Silk Mice/Keyboards to UiHost after construction so the
  UiRoot tree receives live input.
- Tests: 3 new UiRootInputTests covering WantsMouse hit-test, window-drag
  reposition, and non-draggable panel immobility.

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

src/AcDream.App/UI/UiNineSlicePanel.cs

@@ -27,6 +27,7 @@ public sealed class UiNineSlicePanel : UiPanel
         _resolve = resolve;
         BackgroundColor = Vector4.Zero; // suppress the base flat-rect fill
         BorderColor = Vector4.Zero;
+        Draggable = true; // retail windows are movable
     }
 
     /// <summary>