Refactor API client for richer error handling

Refactored CatalogApiClient methods to return ApiResult<T> for create, update, and delete operations, enabling more detailed error reporting. Introduced ApiResult<T> and ProblemDetailsDto types, and added logic to parse and display informative error messages. Updated Catalogs.razor to use the new pattern and show user-friendly error feedback. Added necessary using directives.

DbFirst.BlazorWasm/Pages/Catalogs.razor

@@ -151,9 +151,9 @@ else
             if (isEditing)
             {
                 var updated = await Api.UpdateAsync(editingId, formModel);
-                if (!updated)
+                if (!updated.Success)
                 {
-                    errorMessage = "Aktualisierung fehlgeschlagen.";
+                    errorMessage = updated.Error ?? "Aktualisierung fehlgeschlagen.";
                     return;
                 }
 
@@ -162,9 +162,9 @@ else
             else
             {
                 var created = await Api.CreateAsync(formModel);
-                if (created == null)
+                if (!created.Success || created.Value == null)
                 {
-                    errorMessage = "Anlegen fehlgeschlagen.";
+                    errorMessage = created.Error ?? "Anlegen fehlgeschlagen.";
                     return;
                 }
 
@@ -195,9 +195,9 @@ else
         try
         {
             var deleted = await Api.DeleteAsync(id);
-            if (!deleted)
+            if (!deleted.Success)
             {
-                errorMessage = "Löschen fehlgeschlagen.";
+                errorMessage = deleted.Error ?? "Löschen fehlgeschlagen.";
                 return;
             }