feat(attendance,elective): 实现所有 P2 长期改进项

P2 修复（来自审计报告）：
- 2.4.4: Server Action 错误消息 i18n 化（attendance/elective 全部 Action）
- 2.5.3: 抽取 AttendancePageLayout 组件复用（admin/teacher 页面）
- 2.5.4: 抽取 ElectivePageLayout 组件复用（admin/teacher 列表页）
- 2.6.3: 考勤月历键盘导航（tabIndex + 方向键 + Home/End + role=grid）
- 2.8.2: getStudentAttendanceSummary 分页优化（SQL 聚合统计 + LIMIT 分页）
- 2.8.3: resolveCourseDisplayNames 缓存优化（React cache 去重）
- 2.1.4: elective data-access 跨模块依赖接口抽象（resolvers.ts 可注入）

P2 建议项：
- 选课时间冲突检测（parseSchedule + isScheduleConflict 纯函数 + checkScheduleConflict）
- 学分上限校验（MAX_CREDIT_PER_TERM + checkCreditLimit）
- 考勤/选课数据导出 Excel（export.ts + API 路由扩展）

新增文件：
- src/modules/attendance/components/attendance-page-layout.tsx
- src/modules/elective/components/elective-page-layout.tsx
- src/modules/elective/resolvers.ts
- src/modules/attendance/export.ts
- src/modules/elective/export.ts

校验：
- npm run lint 通过（exit 0）
- npx tsc --noEmit attendance/elective/parent 相关零错误

src/app/(dashboard)/admin/elective/page.tsx

@@ -4,6 +4,7 @@ import { getTranslations } from "next-intl/server"
 
 import { getElectiveCourses } from "@/modules/elective/data-access"
 import { ElectiveCourseList } from "@/modules/elective/components/elective-course-list"
+import { ElectivePageLayout } from "@/modules/elective/components/elective-page-layout"
 import { getSearchParam, type SearchParams } from "@/shared/lib/utils"
 import type { ElectiveCourseStatus } from "@/modules/elective/types"
 
@@ -24,20 +25,23 @@ export default async function AdminElectivePage({
 
   const courses = await getElectiveCourses({ status })
 
+  const header = (
+    <div className="space-y-1">
+      <h2 className="text-2xl font-bold tracking-tight">{t("title.adminList")}</h2>
+      <p className="text-muted-foreground">
+        {t("description.adminList")}
+      </p>
+    </div>
+  )
+
   return (
-    <div className="flex h-full flex-col space-y-8 p-8">
-      <div className="space-y-1">
-        <h2 className="text-2xl font-bold tracking-tight">{t("title.adminList")}</h2>
-        <p className="text-muted-foreground">
-          {t("description.adminList")}
-        </p>
-      </div>
+    <ElectivePageLayout header={header}>
       <ElectiveCourseList
         courses={courses}
         canManage
         createHref="/admin/elective/create"
         editBaseHref="/admin/elective"
       />
-    </div>
+    </ElectivePageLayout>
   )
 }