Fix: wheel over UI scrolls the list instead of zooming the camera

The orbit rig read the wheel unfiltered for zoom, so scrolling a paint-mode
scroll list also zoomed the camera. Gate the zoom behind a shared
InputUtil.PointerOverUi() helper (EventSystem raycast); MeshPainter's private
IsPointerOverUi() now delegates to the same helper (DRY).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Assets/Scripts/Components/MeshPainter/MeshPainter.cs

@@ -1,7 +1,6 @@
 using System;
 using System.Collections.Generic;
 using UnityEngine;
-using UnityEngine.EventSystems;
 #if ENABLE_INPUT_SYSTEM
 using UnityEngine.InputSystem;
 #endif
@@ -1123,11 +1122,7 @@ public partial class MeshPainter : MonoBehaviour
     }
 
     /// <summary>True if the pointer is currently over an interactive UI element (raycast target).</summary>
-    private static bool IsPointerOverUi()
-    {
-        var es = EventSystem.current;
-        return es && es.IsPointerOverGameObject();
-    }
+    private static bool IsPointerOverUi() => InputUtil.PointerOverUi();
 
     private void OnDestroy()
     {