refactor: item 29 cleanup

F16 F17 F18 F19 F20 F21: import polling backs off and pauses in a hidden tab, one date format module, fake timers in the import tests, error codes instead of raw messages, one safe storage wrapper, dead code.

src/pages/AdminUsersPage.jsx

@@ -19,6 +19,7 @@ import {Input} from '@/components/ui/input';
 import {Label} from '@/components/ui/label';
 import PageContainer from '../components/PageContainer.jsx';
 import {deleteUser, fetchUsers, setUserBlocked} from '../utils/admin.js';
+import {formatDate} from '../utils/format.js';
 import useAbortPrevious from '../hooks/useAbortPrevious.js';
 
 const PAGE_SIZE = 20;
@@ -28,7 +29,7 @@ const REFUSAL_KEY = {
     ALREADY_DELETED: 'ADMIN.ERROR_ALREADY_DELETED',
 };
 
-const formatDate = (value, locale) => (value ? new Date(value).toLocaleDateString(locale) : null);
+const formatDateOrNull = (value, locale) => (value ? formatDate(value, locale) : null);
 
 /**
  * PLATFORM_ADMIN only: list, block and delete users. Auth checks the role; a 403 shows "no access".
@@ -140,8 +141,8 @@ export default function AdminUsersPage() {
                                         <p className="truncate font-medium">{user.email}</p>
                                         <p className="truncate text-xs text-muted-foreground">
                                             {[user.displayName,
-                                                t('ADMIN.REGISTERED', {date: formatDate(user.registrationDate, i18n.language)}),
-                                                t('ADMIN.LAST_LOGIN', {date: formatDate(user.lastLoginDate, i18n.language) ?? t('ADMIN.NEVER')}),
+                                                t('ADMIN.REGISTERED', {date: formatDateOrNull(user.registrationDate, i18n.language)}),
+                                                t('ADMIN.LAST_LOGIN', {date: formatDateOrNull(user.lastLoginDate, i18n.language) ?? t('ADMIN.NEVER')}),
                                             ].filter(Boolean).join(' · ')}
                                         </p>
                                     </div>