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/tests/JdeScoping.DataSync.Tests/ScheduleCheckerTests.cs

@@ -206,7 +206,7 @@ public class ScheduleCheckerTests
         tasks[0].MinimumDt.ShouldNotBeNull();
 
         // Expected: lastDaily.EndDT - (3 * 1440 min) = lastDaily.EndDT - 3 days
-        var expectedMinimumDt = lastDaily.EndDt.AddMinutes(-3 * 1440);
+        var expectedMinimumDt = lastDaily.EndDt!.Value.AddMinutes(-3 * 1440);
         tasks[0].MinimumDt!.Value.ShouldBe(expectedMinimumDt, TimeSpan.FromSeconds(1));
     }
 
@@ -242,7 +242,7 @@ public class ScheduleCheckerTests
         tasks[0].MinimumDt.ShouldNotBeNull();
 
         // Hourly uses hourly's timestamp and hourly's interval for lookback calculation
-        var expectedMinimumDt = lastHourly.EndDt.AddMinutes(-3 * 60);
+        var expectedMinimumDt = lastHourly.EndDt!.Value.AddMinutes(-3 * 60);
         tasks[0].MinimumDt!.Value.ShouldBe(expectedMinimumDt, TimeSpan.FromSeconds(1));
     }
 
@@ -271,7 +271,7 @@ public class ScheduleCheckerTests
         var tasks = await _sut.GetPendingTasksAsync();
 
         // Assert
-        var expectedMinimumDt = lastDaily.EndDt.AddMinutes(-5 * 1440);
+        var expectedMinimumDt = lastDaily.EndDt!.Value.AddMinutes(-5 * 1440);
         tasks[0].MinimumDt!.Value.ShouldBe(expectedMinimumDt, TimeSpan.FromSeconds(1));
     }