[Menu] Add z-index to MenuProvider (#2772)

src/Core/Components/Menu/FluentMenuProvider.razor.cs

@@ -20,6 +20,7 @@ public partial class FluentMenuProvider : FluentComponentBase
     /// <summary />
     internal string? StyleValue => new StyleBuilder(Style)
         .AddStyle("display", "fixed")   // To prevent the menu from displaying a scrollbar in body
+        .AddStyle("z-index", ZIndex.Menu.ToString())
         .Build();
 
     /// <summary>