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/learn/ExerciseAttempts.vue

@@ -58,6 +58,7 @@ onMounted(async () => {
 
 <template>
   <n-data-table
+    striped
     :loading="loading"
     :columns="columns"
     :data="rows"