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>

pages/admin/users/index(default).phtml

@@ -46,16 +46,7 @@ ob_start();
     <i class="bi bi-trash3-fill"></i> <?php e(t('Delete')); ?>
   </button>
 <?php endif; ?>
-<details class="dropdown" data-tooltip="<?php e(t("Actions")); ?>" data-tooltip-pos="top">
-  <summary role="button" class="outline secondary"><i class="bi bi-three-dots"></i></summary>
-  <ul dir="rtl">
-    <li dir="ltr">
-      <button class="transparent" type="button" data-users-export>
-        <?php e(t('Export CSV')); ?>
-      </button>
-    </li>
-  </ul>
-</details>
+<?php require templatePath('partials/app-list-export-dropdown.phtml'); ?>
 
 <?php if ($canCreateUser): ?>
   <a role="button" class="app-action-success" href="<?php e(requestPathWithReturnTarget('admin/users/create', Request::pathWithQuery())); ?>">
@@ -119,6 +110,7 @@ $pageConfig = [
   'gridSearch' => $searchConfig,
   'filterSchema' => $clientFilterSchema,
   'filterChipMeta' => $filterChipMeta,
+  'exportUrl' => lurl('admin/users/export'),
   'currentUserUuid' => (string) $currentUserUuid,
   'canUpdateUsers' => (bool) $canUpdateUsers,
   'canUpdateSelf' => (bool) $canUpdateSelf,