feat(06-04): update PermissionsViewModel for multi-site global consumption

- Add _hasLocalSiteOverride field to track local user selection
- Override OnGlobalSitesChanged to pre-populate SelectedSites from global sites
- Set _hasLocalSiteOverride=true when user picks sites via site picker dialog
- Reset _hasLocalSiteOverride=false on tenant switch (OnTenantSwitched)

SharepointToolbox/ViewModels/Tabs/PermissionsViewModel.cs

@@ -75,6 +75,12 @@ public partial class PermissionsViewModel : FeatureViewModelBase
 
     public ObservableCollection<SiteInfo> SelectedSites { get; } = new();
 
+    /// <summary>
+    /// True when the user has manually selected sites via the site picker on this tab.
+    /// Prevents global site changes from overwriting the user's local selection.
+    /// </summary>
+    private bool _hasLocalSiteOverride;
+
     // ── Dialog factory (set by View layer — keeps Window out of ViewModel) ──
 
     /// <summary>
@@ -152,6 +158,15 @@ public partial class PermissionsViewModel : FeatureViewModelBase
 
     // ── FeatureViewModelBase implementation ─────────────────────────────────
 
+    protected override void OnGlobalSitesChanged(IReadOnlyList<SiteInfo> sites)
+    {
+        if (_hasLocalSiteOverride) return;
+
+        SelectedSites.Clear();
+        foreach (var site in sites)
+            SelectedSites.Add(site);
+    }
+
     protected override async Task RunOperationAsync(CancellationToken ct, IProgress<OperationProgress> progress)
     {
         var urls = SelectedSites.Count > 0
@@ -214,6 +229,7 @@ public partial class PermissionsViewModel : FeatureViewModelBase
     protected override void OnTenantSwitched(TenantProfile profile)
     {
         _currentProfile = profile;
+        _hasLocalSiteOverride = false;
         Results = new ObservableCollection<PermissionEntry>();
         SiteUrl = string.Empty;
         SelectedSites.Clear();
@@ -288,6 +304,7 @@ public partial class PermissionsViewModel : FeatureViewModelBase
         var dialog = OpenSitePickerDialog.Invoke();
         if (dialog?.ShowDialog() == true && dialog is Views.Dialogs.SitePickerDialog picker)
         {
+            _hasLocalSiteOverride = true;
             SelectedSites.Clear();
             foreach (var site in picker.SelectedUrls)
                 SelectedSites.Add(site);