feat(02-06): implement PermissionsViewModel with multi-site scan and SitePickerDialog
- PermissionsViewModel extends FeatureViewModelBase, implements RunOperationAsync
- Multi-site mode: loops SelectedSites; single-site mode: uses SiteUrl
- ExportCsvCommand and ExportHtmlCommand enabled only when Results.Count > 0
- OpenSitePickerCommand uses dialog factory pattern (Func<Window>?)
- OnTenantSwitched clears Results, SiteUrl, SelectedSites
- Flat ObservableProperty booleans (IncludeInherited, ScanFolders, etc.) build ScanOptions record
- SitePickerDialog XAML: filterable list with CheckBox column, Title, URL columns
- SitePickerDialog code-behind: loads sites on Window.Loaded, exposes SelectedUrls
- ISessionManager interface extracted for testability (SessionManager implements it)
- StartScanAsync_WithMultipleSiteUrls_CallsServiceOncePerUrl test passes (60/60 + 3 skip)
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