feat(06-04): update single-site tab VMs for global site consumption

- StorageViewModel: add OnGlobalSitesChanged, OnSiteUrlChanged, _hasLocalSiteOverride
- SearchViewModel: add OnGlobalSitesChanged, OnSiteUrlChanged, _hasLocalSiteOverride
- DuplicatesViewModel: add OnGlobalSitesChanged, OnSiteUrlChanged, _hasLocalSiteOverride
- FolderStructureViewModel: add OnGlobalSitesChanged, OnSiteUrlChanged, _hasLocalSiteOverride
- All four VMs pre-fill SiteUrl from first global site; local typing sets override flag
- Tenant switch resets _hasLocalSiteOverride in all four VMs

SharepointToolbox/ViewModels/Tabs/SearchViewModel.cs

@@ -19,6 +19,7 @@ public partial class SearchViewModel : FeatureViewModelBase
     private readonly SearchHtmlExportService _htmlExportService;
     private readonly ILogger<FeatureViewModelBase> _logger;
     private TenantProfile? _currentProfile;
+    private bool _hasLocalSiteOverride;
 
     // ── Filter observable properties ─────────────────────────────────────────
 
@@ -73,6 +74,26 @@ public partial class SearchViewModel : FeatureViewModelBase
         ExportHtmlCommand = new AsyncRelayCommand(ExportHtmlAsync, CanExport);
     }
 
+    protected override void OnGlobalSitesChanged(IReadOnlyList<SiteInfo> sites)
+    {
+        if (_hasLocalSiteOverride) return;
+        SiteUrl = sites.Count > 0 ? sites[0].Url : string.Empty;
+    }
+
+    partial void OnSiteUrlChanged(string value)
+    {
+        if (string.IsNullOrWhiteSpace(value))
+        {
+            _hasLocalSiteOverride = false;
+            if (GlobalSites.Count > 0)
+                SiteUrl = GlobalSites[0].Url;
+        }
+        else if (GlobalSites.Count == 0 || value != GlobalSites[0].Url)
+        {
+            _hasLocalSiteOverride = true;
+        }
+    }
+
     protected override async Task RunOperationAsync(CancellationToken ct, IProgress<OperationProgress> progress)
     {
         if (_currentProfile == null)
@@ -119,6 +140,7 @@ public partial class SearchViewModel : FeatureViewModelBase
     protected override void OnTenantSwitched(Core.Models.TenantProfile profile)
     {
         _currentProfile = profile;
+        _hasLocalSiteOverride = false;
         Results = new ObservableCollection<SearchResult>();
         SiteUrl = string.Empty;
         OnPropertyChanged(nameof(CurrentProfile));