feat(06-03): localize GlobalSitesSelectedLabel in MainWindowViewModel

- Replace hardcoded EN strings with TranslationSource.Instance lookups
- Uses toolbar.globalSites.count (formatted) and toolbar.globalSites.none keys
- Follows same pattern as PermissionsViewModel.SitesSelectedLabel

SharepointToolbox/ViewModels/MainWindowViewModel.cs

@@ -43,12 +43,12 @@ public partial class MainWindowViewModel : ObservableRecipient
     public ObservableCollection<SiteInfo> GlobalSelectedSites { get; } = new();
 
     /// <summary>
-    /// Label for toolbar display: "3 site(s) selected" or "No sites selected".
+    /// Label for toolbar display: "3 site(s) selected" or "No sites selected" (localized).
     /// </summary>
     public string GlobalSitesSelectedLabel =>
         GlobalSelectedSites.Count > 0
-            ? $"{GlobalSelectedSites.Count} site(s) selected"
-            : "No sites selected";
+            ? string.Format(Localization.TranslationSource.Instance["toolbar.globalSites.count"], GlobalSelectedSites.Count)
+            : Localization.TranslationSource.Instance["toolbar.globalSites.none"];
 
     public IAsyncRelayCommand ConnectCommand { get; }
     public IAsyncRelayCommand ClearSessionCommand { get; }