refactor(admin/users): migrate CSV export to core primitive

Replace the inline export implementation with the new generic primitive
(core/Service/Export + helpers/export + app-list-export.js). No more
hand-rolled fputcsv loop, header setup, or escape closure duplicated
here — one implementation, tested once, reused everywhere.

- export().php: uses exportRequireGetRequest, exportCapLimit,
  exportResolveFlavor, ExportColumn[], exportSendCsv. Phone/mobile/
  short_dial keep allowSignedNumeric=true so +49 numbers render
  untouched.
- export(none).phtml: deleted — headers + fputcsv now live in the
  core helper.
- index(default).phtml: inline <details class="dropdown"> "Export CSV"
  replaced by the reusable app-list-export-dropdown partial, exposing
  CSV and Excel flavors. exportUrl added to pageConfig.
- app-users-list.js: hand-rolled export-button binding removed in
  favor of initListExport({ gridConfig, exportUrl }).
- admin-users-index.js: forwards config.exportUrl into
  initUsersListPage.

Net effect: ~100 lines of duplicated export plumbing removed from the
users page; users gain the Excel flavor for free.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

web/js/pages/admin-users-index.js

@@ -171,6 +171,7 @@ createListPageModule({
       appBase,
       csrf,
       labels,
+      exportUrl: config.exportUrl,
       buildBulkUrl,
       bulkDownloadForms: {
         'access-pdf': '#users-access-pdf-bulk-form',