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/shared/i18n/messages/en/elective.json

@@ -95,7 +95,19 @@
     "alreadySelected": "You have already selected this course",
     "selectionClosed": "Selection is closed",
     "gradeMismatch": "Your grade does not match the course requirement",
+    "scheduleConflict": "Schedule conflicts with your existing courses",
+    "creditExceeded": "Credit limit exceeded ({current}/{max})",
     "invalidForm": "Invalid form data",
     "unexpected": "Unexpected error"
+  },
+  "messages": {
+    "created": "Elective course created",
+    "updated": "Elective course updated",
+    "deleted": "Elective course deleted",
+    "selectionOpened": "Selection opened",
+    "selectionClosed": "Selection closed",
+    "lotteryCompleted": "Lottery completed: {enrolled} enrolled, {waitlist} waitlisted",
+    "courseDropped": "Course dropped",
+    "ownershipCheckFailed": "Ownership check failed"
   }
 }