Cache layout user to reduce localStorage access

Add a private _cachedLayoutUser field to BandLayoutService and update EnsureLayoutUserAsync to cache the layout user value after first retrieval or generation. This avoids repeated localStorage calls and improves performance.

DbFirst.BlazorWebApp/Services/BandLayoutService.cs

@@ -9,16 +9,22 @@ namespace DbFirst.BlazorWebApp.Services
     {
         private const string LayoutUserStorageKey = "layoutUser";
         private readonly JsonSerializerOptions _jsonOptions = new(JsonSerializerDefaults.Web);
+        private string? _cachedLayoutUser;
 
         public async Task<string> EnsureLayoutUserAsync()
         {
+            if (!string.IsNullOrWhiteSpace(_cachedLayoutUser))
+                return _cachedLayoutUser;
+
             var layoutUser = await jsRuntime.InvokeAsync<string?>("localStorage.getItem", LayoutUserStorageKey);
             if (string.IsNullOrWhiteSpace(layoutUser))
             {
                 layoutUser = Guid.NewGuid().ToString("N");
                 await jsRuntime.InvokeVoidAsync("localStorage.setItem", LayoutUserStorageKey, layoutUser);
             }
-            return layoutUser;
+
+            _cachedLayoutUser = layoutUser;
+            return _cachedLayoutUser;
         }
 
         public async Task<BandLayout> LoadBandLayoutAsync(