feat(06-03): add global site picker button and count label to toolbar

- Add Separator + Select Sites button (bound to OpenGlobalSitePickerCommand) to ToolBar
- Add TextBlock bound to GlobalSitesSelectedLabel for site count display
- Wire viewModel.OpenGlobalSitePickerDialog factory in MainWindow.xaml.cs using DI
- Add using SharepointToolbox.Core.Models for TenantProfile in code-behind

SharepointToolbox/MainWindow.xaml.cs

@@ -1,6 +1,7 @@
 using System.Windows;
 using System.Windows.Controls;
 using Microsoft.Extensions.DependencyInjection;
+using SharepointToolbox.Core.Models;
 using SharepointToolbox.ViewModels;
 using SharepointToolbox.Views.Dialogs;
 using SharepointToolbox.Views.Tabs;
@@ -20,6 +21,13 @@ public partial class MainWindow : Window
         // Wire profile management dialog factory
         viewModel.OpenProfileManagementDialog = () => serviceProvider.GetRequiredService<ProfileManagementDialog>();
 
+        // Wire global site picker dialog factory (same pattern as PermissionsView)
+        viewModel.OpenGlobalSitePickerDialog = () =>
+        {
+            var factory = serviceProvider.GetRequiredService<Func<TenantProfile, SitePickerDialog>>();
+            return factory(viewModel.SelectedProfile ?? new TenantProfile());
+        };
+
         // Replace Permissions tab placeholder with the DI-resolved PermissionsView
         PermissionsTabItem.Content = serviceProvider.GetRequiredService<PermissionsView>();