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/UiElement.cs

@@ -88,6 +88,11 @@ public abstract class UiElement
     /// <summary>Painter's-algorithm z-order within siblings. Higher = on top.</summary>
     public int  ZOrder          { get; set; }
 
+    /// <summary>If true, a left-drag on this element (or a non-draggable child of
+    /// it) repositions it as a movable window. Intended for top-level panels,
+    /// whose Left/Top are screen coordinates (Root sits at the origin).</summary>
+    public bool Draggable { get; set; }
+
     // ── Tree structure ──────────────────────────────────────────────────
     public UiElement? Parent { get; private set; }