Refactor MassDataGrid editing and validation logic

- Switch to custom edit model (MassDataEditModel) for popup editing, enabling granular field validation and UI control
- Replace default editors with explicit DxTextBox/DxCheckBox bindings
- Add AmountText field for string input and validation; validate and convert in OnEditModelSaving
- Implement duplicate customer check via new GetByCustomerNameAsync API method
- Show ValidationSummary in popup; manage validation state with ValidationMessageStore and EditContext
- Make popup header and width dynamic; show procedure ComboBox only for existing records
- Restore "New" button in grid command column
- Refactor CatalogsGrid to handle validation clearing in OnEditFieldChanged instead of OnTitleChanged
- General improvements to real-time validation feedback

DbFirst.BlazorWebApp/Components/CatalogsGrid.razor

@@ -97,10 +97,7 @@ else
                 @{ SetEditContext(editFormContext.EditContext); var editModel = (CatalogEditModel)editFormContext.EditModel; SetPopupHeaderText(editModel.IsNew); }
                 <DxFormLayout ColCount="2">
                     <DxFormLayoutItem Caption="Titel">
-                        <DxTextBox @bind-Text="editModel.CatTitle"
-                                   @bind-Text:after="OnTitleChanged"
-                                   BindValueMode="BindValueMode.OnInput"
-                                   Width="100%" />
+                        <DxTextBox @bind-Text="editModel.CatTitle" Width="100%" />
                     </DxFormLayoutItem>
                     <DxFormLayoutItem Caption="Kennung">
                         <DxTextBox @bind-Text="editModel.CatString" Width="100%" />
@@ -174,19 +171,15 @@ else
         {
             validationMessageStore.Clear();
             editContext.NotifyValidationStateChanged();
-        }
-    }
-
-    private void OnTitleChanged()
-    {
-        if (validationMessageStore == null || editContext == null)
-        {
             return;
         }
 
-        var field = new FieldIdentifier(editContext.Model, nameof(CatalogEditModel.CatTitle));
-        validationMessageStore.Clear(field);
-        editContext.NotifyValidationStateChanged();
+        if (e.FieldIdentifier.FieldName == nameof(CatalogEditModel.CatTitle))
+        {
+            var field = new FieldIdentifier(editContext.Model, nameof(CatalogEditModel.CatTitle));
+            validationMessageStore.Clear(field);
+            editContext.NotifyValidationStateChanged();
+        }
     }
 
     private void SetPopupHeaderText(bool isNew)