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.DataSync/Services/ScheduleChecker.cs

@@ -86,7 +86,7 @@ public class ScheduleChecker : IScheduleChecker
             _logger.LogDebug(
                 "Mass sync needed for {Table}: last={LastSync}, interval={Interval}m",
                 config.TableName,
-                lastMass?.EndDt.ToString("o") ?? "never",
+                lastMass?.EndDt?.ToString("o") ?? "never",
                 config.MassConfig.IntervalMinutes);
 
             return CreateTask(config, UpdateTypes.Mass, null);
@@ -100,7 +100,7 @@ public class ScheduleChecker : IScheduleChecker
             _logger.LogDebug(
                 "Daily sync needed for {Table}: last={LastSync}, interval={Interval}m, minDT={MinDT}",
                 config.TableName,
-                lastDaily?.EndDt.ToString("o") ?? "never",
+                lastDaily?.EndDt?.ToString("o") ?? "never",
                 config.DailyConfig.IntervalMinutes,
                 minimumDt?.ToString("o") ?? "null");
 
@@ -115,7 +115,7 @@ public class ScheduleChecker : IScheduleChecker
             _logger.LogDebug(
                 "Hourly sync needed for {Table}: last={LastSync}, interval={Interval}m, minDT={MinDT}",
                 config.TableName,
-                lastHourly?.EndDt.ToString("o") ?? "never",
+                lastHourly?.EndDt?.ToString("o") ?? "never",
                 config.HourlyConfig.IntervalMinutes,
                 minimumDt?.ToString("o") ?? "null");
 
@@ -143,7 +143,8 @@ public class ScheduleChecker : IScheduleChecker
             return true;
         }
 
-        var nextSyncDue = lastMass.EndDt.AddMinutes(config.MassConfig.IntervalMinutes);
+        // EndDt is set for successful syncs (GetLastDataUpdatesAsync filters WasSuccessful=1)
+        var nextSyncDue = lastMass.EndDt!.Value.AddMinutes(config.MassConfig.IntervalMinutes);
         return now > nextSyncDue;
     }
 
@@ -170,7 +171,8 @@ public class ScheduleChecker : IScheduleChecker
             return true;
         }
 
-        var nextSyncDue = lastDaily.EndDt.AddMinutes(config.DailyConfig.IntervalMinutes);
+        // EndDt is set for successful syncs (GetLastDataUpdatesAsync filters WasSuccessful=1)
+        var nextSyncDue = lastDaily.EndDt!.Value.AddMinutes(config.DailyConfig.IntervalMinutes);
         return now > nextSyncDue;
     }
 
@@ -202,7 +204,8 @@ public class ScheduleChecker : IScheduleChecker
             return true;
         }
 
-        var nextSyncDue = lastHourly.EndDt.AddMinutes(config.HourlyConfig.IntervalMinutes);
+        // EndDt is set for successful syncs (GetLastDataUpdatesAsync filters WasSuccessful=1)
+        var nextSyncDue = lastHourly.EndDt!.Value.AddMinutes(config.HourlyConfig.IntervalMinutes);
         return now > nextSyncDue;
     }
 
@@ -217,7 +220,8 @@ public class ScheduleChecker : IScheduleChecker
         }
 
         var lookbackMinutes = _options.Value.LookbackMultiplier * intervalMinutes;
-        return lastUpdate.EndDt.AddMinutes(-lookbackMinutes);
+        // EndDt is set for successful syncs (GetLastDataUpdatesAsync filters WasSuccessful=1)
+        return lastUpdate.EndDt!.Value.AddMinutes(-lookbackMinutes);
     }
 
     /// <summary>