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.Core/Models/Infrastructure/DataUpdate.cs

@@ -33,9 +33,9 @@ public class DataUpdate
     public DateTime StartDt { get; set; }
 
     /// <summary>
-    /// Timestamp at end of update
+    /// Timestamp at end of update (null while sync is in progress)
     /// </summary>
-    public DateTime EndDt { get; set; }
+    public DateTime? EndDt { get; set; }
 
     /// <summary>
     /// Type of data update (Hourly, Daily, Mass)