feat(06-02): add global site selection state, command, and broadcast to MainWindowViewModel

- Add OpenGlobalSitePickerDialog factory property (dialog factory pattern)
- Add GlobalSelectedSites ObservableCollection<SiteInfo>
- Add GlobalSitesSelectedLabel computed property for toolbar display
- Add OpenGlobalSitePickerCommand (disabled when no profile selected)
- Broadcast GlobalSitesChangedMessage via WeakReferenceMessenger on collection change
- Clear GlobalSelectedSites on tenant switch (OnSelectedProfileChanged)
- Clear GlobalSelectedSites on session clear (ClearSessionAsync)
- Add using SharepointToolbox.Views.Dialogs for SitePickerDialog cast

SharepointToolbox/ViewModels/MainWindowViewModel.cs

@@ -7,6 +7,7 @@ using Microsoft.Extensions.Logging;
 using SharepointToolbox.Core.Messages;
 using SharepointToolbox.Core.Models;
 using SharepointToolbox.Services;
+using SharepointToolbox.Views.Dialogs;
 
 namespace SharepointToolbox.ViewModels;
 
@@ -19,6 +20,12 @@ public partial class MainWindowViewModel : ObservableRecipient
     // Set by the view layer (MainWindow.xaml.cs) to open the dialog using DI
     public Func<Window>? OpenProfileManagementDialog { get; set; }
 
+    /// <summary>
+    /// Factory set by MainWindow.xaml.cs to open the SitePickerDialog for global site selection.
+    /// Returns the opened Window; ViewModel calls ShowDialog() on it.
+    /// </summary>
+    public Func<Window>? OpenGlobalSitePickerDialog { get; set; }
+
     [ObservableProperty]
     private TenantProfile? _selectedProfile;
 
@@ -33,9 +40,20 @@ public partial class MainWindowViewModel : ObservableRecipient
 
     public ObservableCollection<TenantProfile> TenantProfiles { get; } = new();
 
+    public ObservableCollection<SiteInfo> GlobalSelectedSites { get; } = new();
+
+    /// <summary>
+    /// Label for toolbar display: "3 site(s) selected" or "No sites selected".
+    /// </summary>
+    public string GlobalSitesSelectedLabel =>
+        GlobalSelectedSites.Count > 0
+            ? $"{GlobalSelectedSites.Count} site(s) selected"
+            : "No sites selected";
+
     public IAsyncRelayCommand ConnectCommand { get; }
     public IAsyncRelayCommand ClearSessionCommand { get; }
     public RelayCommand ManageProfilesCommand { get; }
+    public RelayCommand OpenGlobalSitePickerCommand { get; }
 
     public MainWindowViewModel(
         ProfileService profileService,
@@ -49,6 +67,12 @@ public partial class MainWindowViewModel : ObservableRecipient
         ConnectCommand = new AsyncRelayCommand(ConnectAsync, () => SelectedProfile != null);
         ClearSessionCommand = new AsyncRelayCommand(ClearSessionAsync, () => SelectedProfile != null);
         ManageProfilesCommand = new RelayCommand(OpenProfileManagement);
+        OpenGlobalSitePickerCommand = new RelayCommand(ExecuteOpenGlobalSitePicker, () => SelectedProfile != null);
+        GlobalSelectedSites.CollectionChanged += (_, _) =>
+        {
+            OnPropertyChanged(nameof(GlobalSitesSelectedLabel));
+            BroadcastGlobalSites();
+        };
 
         IsActive = true;
     }
@@ -74,6 +98,9 @@ public partial class MainWindowViewModel : ObservableRecipient
         }
         ConnectCommand.NotifyCanExecuteChanged();
         ClearSessionCommand.NotifyCanExecuteChanged();
+        // Clear global site selection on tenant switch (sites belong to a tenant)
+        GlobalSelectedSites.Clear();
+        OpenGlobalSitePickerCommand.NotifyCanExecuteChanged();
     }
 
     public async Task LoadProfilesAsync()
@@ -116,6 +143,7 @@ public partial class MainWindowViewModel : ObservableRecipient
         try
         {
             await _sessionManager.ClearSessionAsync(SelectedProfile.TenantUrl);
+            GlobalSelectedSites.Clear();
             ConnectionStatus = "Not connected";
         }
         catch (Exception ex)
@@ -134,4 +162,22 @@ public partial class MainWindowViewModel : ObservableRecipient
         // Reload profiles after dialog closes (modal — ShowDialog blocks until closed)
         _ = LoadProfilesAsync();
     }
+
+    private void ExecuteOpenGlobalSitePicker()
+    {
+        if (OpenGlobalSitePickerDialog == null) return;
+        var dialog = OpenGlobalSitePickerDialog.Invoke();
+        if (dialog?.ShowDialog() == true && dialog is SitePickerDialog picker)
+        {
+            GlobalSelectedSites.Clear();
+            foreach (var site in picker.SelectedUrls)
+                GlobalSelectedSites.Add(site);
+        }
+    }
+
+    private void BroadcastGlobalSites()
+    {
+        WeakReferenceMessenger.Default.Send(
+            new GlobalSitesChangedMessage(GlobalSelectedSites.ToList().AsReadOnly()));
+    }
 }