Fix tab switching by removing field shadowing in Edit.razor.cs

The Edit component was declaring its own private fields for activeTab, errorMessage, and successMessage, which shadowed the protected fields from CustomComponentBase. This prevented SetActiveTab() from working correctly since it updated the base class field but the UI read from the local field.

Co-authored-by: StuartFerguson <16325469+StuartFerguson@users.noreply.github.com>

Author: Copilot

EstateManagementUI.BlazorServer/Components/Pages/Merchants/Edit.razor.cs

@@ -20,9 +20,6 @@ public partial class Edit {
         private MerchantModels.MerchantModel? merchant;
         private bool isLoading = true;
         private bool isSaving = false;
-        private string? errorMessage;
-        private string? successMessage;
-        private string activeTab = "details";
 
         // Unified model for editing
         private MerchantModels.MerchantEditModel merchantEditModel = new();
@@ -49,6 +46,11 @@ public partial class Edit {
         private bool showAddDevice = false;
         private string? deviceIdentifier;
 
+        protected override async Task OnInitializedAsync() {
+            await base.OnInitializedAsync();
+            this.SetActiveTab("details");
+        }
+
         protected override async Task OnAfterRenderAsync(bool firstRender) {
             if (!firstRender) {
                 return;