style(后台): 后台各张列表都改成斑马底纹（题目、用户、比赛、公告、练一练记录、标签、AI 报告、成就）

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

apps/web/src/admin/problem/list.vue

@@ -348,6 +348,7 @@ watch(() => [query.page, query.limit, query.author, query.kind, query.visible],
     <n-button size="small" quaternary @click="selectedRowKeys = []">取消选择</n-button>
   </div>
   <n-data-table
+    striped
     class="problem-table"
     :columns="columns"
     :data="problems"