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/Services/MassDataApiClient.cs

@@ -32,4 +32,21 @@ public class MassDataApiClient
         var payload = await response.Content.ReadFromJsonAsync<MassDataReadDto>();
         return payload ?? new MassDataReadDto();
     }
+
+    public async Task<MassDataReadDto?> GetByCustomerNameAsync(string customerName)
+    {
+        if (string.IsNullOrWhiteSpace(customerName))
+        {
+            return null;
+        }
+
+        var response = await _httpClient.GetAsync($"{Endpoint}/{Uri.EscapeDataString(customerName)}");
+        if (response.StatusCode == System.Net.HttpStatusCode.NotFound)
+        {
+            return null;
+        }
+
+        response.EnsureSuccessStatusCode();
+        return await response.Content.ReadFromJsonAsync<MassDataReadDto>();
+    }
 }