feat(06-01): extend FeatureViewModelBase with GlobalSites support

- Add protected GlobalSites property (IReadOnlyList<SiteInfo>) initialized to Array.Empty
- Register GlobalSitesChangedMessage in OnActivated alongside TenantSwitchedMessage
- Add private OnGlobalSitesReceived to update GlobalSites and invoke virtual hook
- Add protected virtual OnGlobalSitesChanged for derived VMs to override
- [Rule 3 - Blocking] Fix MainWindowViewModel missing ExecuteOpenGlobalSitePicker and BroadcastGlobalSites stubs referenced in constructor (pre-existing partial state from earlier TODO commit)

SharepointToolbox/ViewModels/FeatureViewModelBase.cs

@@ -23,6 +23,12 @@ public abstract partial class FeatureViewModelBase : ObservableRecipient
     [ObservableProperty]
     private int _progressValue;
 
+    /// <summary>
+    /// Sites selected in the global toolbar picker. Updated via GlobalSitesChangedMessage.
+    /// Derived VMs check this in RunOperationAsync before falling back to per-tab SiteUrl.
+    /// </summary>
+    protected IReadOnlyList<SiteInfo> GlobalSites { get; private set; } = Array.Empty<SiteInfo>();
+
     public IAsyncRelayCommand RunCommand { get; }
     public RelayCommand CancelCommand { get; }
 
@@ -73,10 +79,26 @@ public abstract partial class FeatureViewModelBase : ObservableRecipient
     protected override void OnActivated()
     {
         Messenger.Register<TenantSwitchedMessage>(this, (r, m) => ((FeatureViewModelBase)r).OnTenantSwitched(m.Value));
+        Messenger.Register<GlobalSitesChangedMessage>(this, (r, m) => ((FeatureViewModelBase)r).OnGlobalSitesReceived(m.Value));
     }
 
     protected virtual void OnTenantSwitched(TenantProfile profile)
     {
         // Derived classes override to reset their state
     }
+
+    private void OnGlobalSitesReceived(IReadOnlyList<SiteInfo> sites)
+    {
+        GlobalSites = sites;
+        OnGlobalSitesChanged(sites);
+    }
+
+    /// <summary>
+    /// Called when the global site selection changes. Override in derived VMs
+    /// to update UI state (e.g., pre-fill SiteUrl from first global site).
+    /// </summary>
+    protected virtual void OnGlobalSitesChanged(IReadOnlyList<SiteInfo> sites)
+    {
+        // Derived classes override to react to global site changes
+    }
 }