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/attendance/page.tsx

@@ -14,6 +14,7 @@ import { getAttendanceRecords, getAttendanceStats } from "@/modules/attendance/d
 import { AttendanceFilters } from "@/modules/attendance/components/attendance-filters"
 import { AttendanceStatsCards } from "@/modules/attendance/components/attendance-stats-cards"
 import { AttendanceRecordList } from "@/modules/attendance/components/attendance-record-list"
+import { AttendancePageLayout } from "@/modules/attendance/components/attendance-page-layout"
 import type { AttendanceStatus } from "@/modules/attendance/types"
 
 export const dynamic = "force-dynamic"
@@ -55,25 +56,27 @@ export default async function AdminAttendancePage({
     date: date && date.length > 0 ? date : undefined,
   })
 
-  return (
-    <div className="h-full flex-1 flex-col space-y-8 p-8 md:flex">
-      <div className="flex items-center justify-between space-y-2">
-        <div>
-          <h2 className="text-2xl font-bold tracking-tight">{t("title.adminOverview")}</h2>
-          <p className="text-muted-foreground">{t("description.adminOverview")}</p>
-        </div>
-        <Button asChild variant="outline">
-          <Link href="/teacher/attendance/stats">
-            <BarChart3 className="mr-2 h-4 w-4" />
-            {t("actions.stats")}
-          </Link>
-        </Button>
+  const header = (
+    <div className="flex items-center justify-between space-y-2">
+      <div>
+        <h2 className="text-2xl font-bold tracking-tight">{t("title.adminOverview")}</h2>
+        <p className="text-muted-foreground">{t("description.adminOverview")}</p>
       </div>
+      <Button asChild variant="outline">
+        <Link href="/teacher/attendance/stats">
+          <BarChart3 className="mr-2 h-4 w-4" />
+          {t("actions.stats")}
+        </Link>
+      </Button>
+    </div>
+  )
 
-      <AttendanceStatsCards stats={stats} />
-
-      <AttendanceFilters classes={classOptions} />
-
+  return (
+    <AttendancePageLayout
+      header={header}
+      stats={<AttendanceStatsCards stats={stats} />}
+      filters={<AttendanceFilters classes={classOptions} />}
+    >
       {result.items.length === 0 && !classId && !status && !date ? (
         <EmptyState
           title={t("list.empty")}
@@ -83,6 +86,6 @@ export default async function AdminAttendancePage({
       ) : (
         <AttendanceRecordList records={result.items} />
       )}
-    </div>
+    </AttendancePageLayout>
   )
 }