Refactor UpsertAsync to use ApiResult for error handling

MassDataApiClient.UpsertAsync now returns ApiResult to standardize success and error reporting, including detailed error extraction from API responses. Updated MassDataGrid.razor to handle the new result type and display error messages accordingly. Removed obsolete try-catch logic in favor of the new pattern.

DbFirst.BlazorWebApp/Services/MassDataApiClient.cs

@@ -36,12 +36,34 @@ public class MassDataApiClient
         return result ?? new List<MassDataReadDto>();
     }
 
-    public async Task<MassDataReadDto> UpsertAsync(MassDataWriteDto dto)
+    public async Task<ApiResult<MassDataReadDto?>> UpsertAsync(MassDataWriteDto dto)
     {
         var response = await _httpClient.PostAsJsonAsync($"{Endpoint}/upsert", dto);
-        response.EnsureSuccessStatusCode();
-        var payload = await response.Content.ReadFromJsonAsync<MassDataReadDto>();
-        return payload ?? new MassDataReadDto();
+        if (response.IsSuccessStatusCode)
+        {
+            var payload = await response.Content.ReadFromJsonAsync<MassDataReadDto>();
+            return ApiResult<MassDataReadDto?>.Ok(payload);
+        }
+
+        var error = await ReadErrorAsync(response);
+        return ApiResult<MassDataReadDto?>.Fail(error);
+    }
+
+    private static async Task<string> ReadErrorAsync(HttpResponseMessage response)
+    {
+        try
+        {
+            var problem = await response.Content.ReadFromJsonAsync<ProblemDetailsDto>();
+            if (problem != null && !string.IsNullOrWhiteSpace(problem.Title))
+                return problem.Detail ?? problem.Title ?? response.ReasonPhrase ?? "Unbekannter Fehler";
+        }
+        catch { }
+
+        var body = await response.Content.ReadAsStringAsync();
+        if (!string.IsNullOrWhiteSpace(body))
+            return body;
+
+        return $"Fehler {(int)response.StatusCode} {response.ReasonPhrase}".Trim();
     }
 
     public async Task<MassDataReadDto?> GetByCustomerNameAsync(string customerName)