refactor(securestoremanager): add platform service abstractions and constants

Implement deferred code review findings:
- Add IDialogService/IClipboardService interfaces for testable platform operations
- Create AvaloniaDialogService and AvaloniaClipboardService implementations
- Extract dialog strings and file extensions to centralized Constants classes
- Refactor ViewModels to use DI instead of event delegates
- Update tests to use mock services

NEW/src/JdeScoping.Api/Controllers/PipelineController.cs

@@ -84,7 +84,7 @@ public class PipelineController : ApiControllerBase
             var successKey = $"{tableName}_{(int)updateType}";
             lastSuccessful.TryGetValue(successKey, out var lastSuccess);
 
-            var nextRequired = lastSuccess?.EndDt.AddMinutes(interval);
+            var nextRequired = lastSuccess?.EndDt?.AddMinutes(interval);
             var isOverdue = DataUpdateRepository.IsOverdue(
                 lastSuccess?.EndDt, tableName, updateType, null);