refactor(attendance,elective): 审计第二轮 — 全量完成 P0/P1 改进项

P0 修复：
- 页面层 i18n 全量补齐（admin/teacher/parent/student × attendance/elective）
- types.ts 状态标签常量迁移至 constants.ts（i18n key + Badge variant）
- 修复 getTranslations 导入路径（next-intl → next-intl/server）

P1 改进：
- 解耦 parent 模块对 attendance 类型的直接依赖（本地 view-model 类型）
- 导出纯函数（computeStats/buildWarnings/buildLotteryRankCase 等）
- 统一空状态为 EmptyState 组件
- 清理死代码读 Action（attendance 5 个 + elective 3 个）
- 预留监控埋点接口（trackEvent 13 个新事件名）
- 补齐骨架屏 loading.tsx（8 个页面）
- AlertDialog 替换 window.confirm（student-selection-view）
- a11y 改进（aria-label/role/键盘导航）

修复：
- AttendanceStatus 从 constants.ts 重导出，消除 types/constants 双源混乱
- buildWarnings 的 Translator 类型改用 ReturnType<typeof useTranslations>

src/app/(dashboard)/teacher/attendance/sheet/loading.tsx

@@ -0,0 +1,27 @@
+import { Card, CardContent, CardHeader } from "@/shared/components/ui/card"
+import { Skeleton } from "@/shared/components/ui/skeleton"
+
+export default function Loading() {
+  return (
+    <div className="h-full flex-1 flex-col space-y-8 p-8 md:flex">
+      <div className="space-y-2">
+        <Skeleton className="h-8 w-48" />
+        <Skeleton className="h-4 w-72" />
+      </div>
+      <Card>
+        <CardHeader className="space-y-2">
+          <Skeleton className="h-5 w-32" />
+        </CardHeader>
+        <CardContent className="space-y-3">
+          <div className="flex gap-3">
+            <Skeleton className="h-9 w-48" />
+            <Skeleton className="h-9 w-40" />
+          </div>
+          {Array.from({ length: 8 }).map((_, i) => (
+            <Skeleton key={i} className="h-12 w-full" />
+          ))}
+        </CardContent>
+      </Card>
+    </div>
+  )
+}

src/app/(dashboard)/teacher/attendance/sheet/page.tsx

@@ -1,4 +1,5 @@
 import type { JSX } from "react"
+import { getTranslations } from "next-intl/server"
 import { getTeacherClasses } from "@/modules/classes/data-access"
 import { getClassStudentsForAttendance } from "@/modules/attendance/data-access"
 import { AttendanceSheet } from "@/modules/attendance/components/attendance-sheet"
@@ -11,6 +12,7 @@ export default async function AttendanceSheetPage({
 }: {
   searchParams: Promise<SearchParams>
 }): Promise<JSX.Element> {
+  const t = await getTranslations("attendance")
   const sp = await searchParams
 
   const defaultClassId = getParam(sp, "classId")
@@ -27,10 +29,8 @@ export default async function AttendanceSheetPage({
   return (
     <div className="h-full flex-1 flex-col space-y-8 p-8 md:flex">
       <div>
-        <h1 className="text-2xl font-bold tracking-tight">Take Attendance</h1>
-        <p className="text-muted-foreground">
-          Select a class and date, then mark attendance for each student.
-        </p>
+        <h1 className="text-2xl font-bold tracking-tight">{t("title.sheet")}</h1>
+        <p className="text-muted-foreground">{t("sheet.description")}</p>
       </div>
 
       <AttendanceSheet