feat(06-04): update TransferViewModel for global site consumption; confirm BulkMembers excluded

- TransferViewModel: add _hasLocalSourceSiteOverride field
- Override OnGlobalSitesChanged to pre-fill SourceSiteUrl from first global site
- Add OnSourceSiteUrlChanged partial to detect local user input
- Reset _hasLocalSourceSiteOverride on tenant switch
- BulkMembersViewModel confirmed excluded: no SiteUrl field, CSV-driven, no OnGlobalSitesChanged override added

SharepointToolbox/ViewModels/Tabs/TransferViewModel.cs

@@ -17,6 +17,7 @@ public partial class TransferViewModel : FeatureViewModelBase
     private readonly BulkResultCsvExportService _exportService;
     private readonly ILogger<FeatureViewModelBase> _logger;
     private TenantProfile? _currentProfile;
+    private bool _hasLocalSourceSiteOverride;
 
     // Source selection
     [ObservableProperty] private string _sourceSiteUrl = string.Empty;
@@ -61,6 +62,26 @@ public partial class TransferViewModel : FeatureViewModelBase
         ExportFailedCommand = new AsyncRelayCommand(ExportFailedAsync, () => HasFailures);
     }
 
+    protected override void OnGlobalSitesChanged(IReadOnlyList<SiteInfo> sites)
+    {
+        if (_hasLocalSourceSiteOverride) return;
+        SourceSiteUrl = sites.Count > 0 ? sites[0].Url : string.Empty;
+    }
+
+    partial void OnSourceSiteUrlChanged(string value)
+    {
+        if (string.IsNullOrWhiteSpace(value))
+        {
+            _hasLocalSourceSiteOverride = false;
+            if (GlobalSites.Count > 0)
+                SourceSiteUrl = GlobalSites[0].Url;
+        }
+        else if (GlobalSites.Count == 0 || value != GlobalSites[0].Url)
+        {
+            _hasLocalSourceSiteOverride = true;
+        }
+    }
+
     protected override async Task RunOperationAsync(CancellationToken ct, IProgress<OperationProgress> progress)
     {
         if (_currentProfile == null)
@@ -152,6 +173,7 @@ public partial class TransferViewModel : FeatureViewModelBase
     protected override void OnTenantSwitched(TenantProfile profile)
     {
         _currentProfile = profile;
+        _hasLocalSourceSiteOverride = false;
         SourceSiteUrl = string.Empty;
         SourceLibrary = string.Empty;
         SourceFolderPath = string.Empty;