feat(frontend): 分页 pageSize 持久化到 localStorage，刷新后自动恢复

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/components/admin/announcements/AnnouncementReadStatusDialog.vue

@@ -69,6 +69,7 @@ import { adminAPI } from '@/api/admin'
 import { formatDateTime } from '@/utils/format'
 import type { AnnouncementUserReadStatus } from '@/types'
 import type { Column } from '@/components/common/types'
+import { getPersistedPageSize } from '@/composables/usePersistedPageSize'
 
 import BaseDialog from '@/components/common/BaseDialog.vue'
 import DataTable from '@/components/common/DataTable.vue'
@@ -92,7 +93,7 @@ const search = ref('')
 
 const pagination = reactive({
   page: 1,
-  page_size: 20,
+  page_size: getPersistedPageSize(),
   total: 0,
   pages: 0
 })