- TransferViewModel: add _hasLocalSourceSiteOverride field
- Override OnGlobalSitesChanged to pre-fill SourceSiteUrl from first global site
- Add OnSourceSiteUrlChanged partial to detect local user input
- Reset _hasLocalSourceSiteOverride on tenant switch
- BulkMembersViewModel confirmed excluded: no SiteUrl field, CSV-driven, no OnGlobalSitesChanged override added