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/Helper/InputUtil.cs

@@ -1,4 +1,5 @@
 using UnityEngine;
+using UnityEngine.EventSystems;
 #if ENABLE_INPUT_SYSTEM
 using UnityEngine.InputSystem;
 #endif
@@ -40,6 +41,15 @@ public static class InputUtil
         Cursor.visible = !locked;
     }
 
+    /// <summary>True while the pointer is over an interactive UI element (a raycast target), so the
+    /// wheel/click belongs to the UI (e.g. a scroll list) and must not also zoom the camera or paint
+    /// the mesh behind it. False when no EventSystem exists.</summary>
+    public static bool PointerOverUi()
+    {
+        var es = EventSystem.current;
+        return es && es.IsPointerOverGameObject();
+    }
+
     public static bool ShiftHeld()
     {
 #if ENABLE_INPUT_SYSTEM