feat(课堂看板): 加「回头看 →」，带着这个班进数据统计看以前几节课

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

apps/web/src/oj/classroom/board.vue

@@ -269,6 +269,13 @@ function submissionsHref(
           >
         </n-text>
         <n-button :loading="loading" @click="load">刷新</n-button>
+        <!-- 看板只管今天；以前几节课的、任意时间段的去数据统计（带上这个班） -->
+        <n-button
+          v-if="!projector"
+          @click="router.push({ name: 'statistics', query: className ? { className } : {} })"
+        >
+          回头看 →
+        </n-button>
         <n-button :type="projector ? 'primary' : 'default'" @click="projector = !projector">
           {{ projector ? "退出投影" : "投影模式" }}
         </n-button>