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/modules/elective/export.ts

@@ -0,0 +1,102 @@
+import "server-only"
+
+import { getTranslations } from "next-intl/server"
+
+import { exportToExcel } from "@/shared/lib/excel"
+
+import { getElectiveCourses } from "./data-access"
+import { getCourseSelections } from "./data-access-selections"
+
+/**
+ * 导出选修课课程列表到 Excel
+ * Sheet 1: 课程明细
+ */
+export async function exportElectiveCoursesToExcel(params: {
+  status?: string
+  teacherId?: string
+}): Promise<Buffer> {
+  const t = await getTranslations("elective")
+
+  const courses = await getElectiveCourses({
+    status: params.status as "draft" | "open" | "closed" | "cancelled" | undefined,
+    teacherId: params.teacherId,
+  })
+
+  const rows = courses.map((c) => ({
+    [t("fields.name")]: c.name,
+    [t("fields.teacher")]: c.teacherName ?? "",
+    [t("fields.subject")]: c.subjectName ?? "",
+    [t("fields.grade")]: c.gradeName ?? "",
+    [t("fields.capacity")]: c.capacity,
+    [t("fields.enrolled")]: c.enrolledCount,
+    [t("fields.classroom")]: c.classroom ?? "",
+    [t("fields.schedule")]: c.schedule ?? "",
+    [t("fields.credit")]: c.credit,
+    [t("fields.selectionMode")]: t(`selectionMode.${c.selectionMode}`),
+    status: t(`status.${c.status}`),
+    [t("fields.startDate")]: c.startDate ?? "",
+    [t("fields.endDate")]: c.endDate ?? "",
+  }))
+
+  return exportToExcel({
+    sheets: [
+      {
+        name: t("title.adminList"),
+        columns: [
+          { header: t("fields.name"), key: t("fields.name"), width: 24 },
+          { header: t("fields.teacher"), key: t("fields.teacher"), width: 16 },
+          { header: t("fields.subject"), key: t("fields.subject"), width: 14 },
+          { header: t("fields.grade"), key: t("fields.grade"), width: 12 },
+          { header: t("fields.capacity"), key: t("fields.capacity"), width: 10 },
+          { header: t("fields.enrolled"), key: t("fields.enrolled"), width: 10 },
+          { header: t("fields.classroom"), key: t("fields.classroom"), width: 14 },
+          { header: t("fields.schedule"), key: t("fields.schedule"), width: 20 },
+          { header: t("fields.credit"), key: t("fields.credit"), width: 8 },
+          { header: t("fields.selectionMode"), key: t("fields.selectionMode"), width: 16 },
+          { header: "Status", key: "status", width: 12 },
+          { header: t("fields.startDate"), key: t("fields.startDate"), width: 14 },
+          { header: t("fields.endDate"), key: t("fields.endDate"), width: 14 },
+        ],
+        rows,
+      },
+    ],
+  })
+}
+
+/**
+ * 导出课程选课名单到 Excel
+ * Sheet 1: 选课名单
+ */
+export async function exportCourseSelectionsToExcel(params: {
+  courseId: string
+}): Promise<Buffer> {
+  const t = await getTranslations("elective")
+
+  const selections = await getCourseSelections(params.courseId)
+
+  const rows = selections.map((s, idx) => ({
+    "#": idx + 1,
+    [t("fields.name")]: s.studentName ?? "",
+    status: t(`selectionStatus.${s.status}`),
+    priority: s.priority ?? 1,
+    selectedAt: s.selectedAt.split("T")[0],
+    enrolledAt: s.enrolledAt ? s.enrolledAt.split("T")[0] : "",
+  }))
+
+  return exportToExcel({
+    sheets: [
+      {
+        name: t("student.mySelections"),
+        columns: [
+          { header: "#", key: "#", width: 6 },
+          { header: t("fields.name"), key: t("fields.name"), width: 18 },
+          { header: "Status", key: "status", width: 12 },
+          { header: "Priority", key: "priority", width: 10 },
+          { header: "Selected At", key: "selectedAt", width: 14 },
+          { header: "Enrolled At", key: "enrolledAt", width: 14 },
+        ],
+        rows,
+      },
+    ],
+  })
+}