refactor(dashboard): V2 审计重构 — i18n 补齐 + 共享抽象 + 单测 + a11y

V2 审计报告（docs/architecture/audit/dashboard-audit-report-v2.md）发现并修复：

- P0 i18n：10 个子组件硬编码字符串全部接入 next-intl（teacher-quick-actions /
  teacher-classes-card / teacher-homework-card / teacher-schedule /
  recent-submissions / teacher-grade-trends / student-grades-card /
  student-today-schedule-card / student-upcoming-assignments-card /
  admin-dashboard），新增 ~50 个翻译键
- P1 共享抽象：新增 DashboardGreetingHeader 组件，消除 teacher/student
  头部 90% 重复代码，两个 Header 改为薄包装
- P2 单测：为 6 个纯函数添加 31 个单元测试
  （tests/integration/dashboard/dashboard-utils.test.ts）
- P2 a11y：admin 表格 caption、teacher/student 视图语义化标签
  （header / section aria-label / aside aria-label）
- 同步架构图 004/005

src/modules/dashboard/components/teacher-dashboard/teacher-classes-card.tsx

@@ -1,30 +1,33 @@
 import Link from "next/link"
 import { Users } from "lucide-react"
+import { getTranslations } from "next-intl/server"
 
 import { Button } from "@/shared/components/ui/button"
 import { Card, CardContent, CardHeader, CardTitle } from "@/shared/components/ui/card"
 import { EmptyState } from "@/shared/components/ui/empty-state"
 import type { TeacherClass } from "@/modules/classes/types"
 
-export function TeacherClassesCard({ classes }: { classes: TeacherClass[] }) {
+export async function TeacherClassesCard({ classes }: { classes: TeacherClass[] }) {
+  const t = await getTranslations("dashboard")
+
   return (
     <Card>
       <CardHeader className="flex flex-row items-center justify-between">
         <CardTitle className="text-base flex items-center gap-2">
           <Users className="h-4 w-4 text-muted-foreground" />
-          My Classes
+          {t("sections.myClasses")}
         </CardTitle>
         <Button asChild variant="outline" size="sm">
-          <Link href="/teacher/classes/my">View all</Link>
+          <Link href="/teacher/classes/my">{t("quickActions.viewAll")}</Link>
         </Button>
       </CardHeader>
       <CardContent className="grid gap-3">
         {classes.length === 0 ? (
           <EmptyState
             icon={Users}
-            title="No classes yet"
-            description="Create a class to start managing students and schedules."
-            action={{ label: "Create class", href: "/teacher/classes/my" }}
+            title={t("empty.noClassesYet")}
+            description={t("empty.noClassesDesc")}
+            action={{ label: t("quickActions.createClass"), href: "/teacher/classes/my" }}
             className="border-none h-72"
           />
         ) : (
@@ -44,13 +47,13 @@ export function TeacherClassesCard({ classes }: { classes: TeacherClass[] }) {
                     {c.homeroom && (
                         <>
                             <span>·</span>
-                            <span>Homeroom: {c.homeroom}</span>
+                            <span>{t("schedule.homeroom")}: {c.homeroom}</span>
                         </>
                     )}
                     {c.room && (
                         <>
                             <span>·</span>
-                            <span>Room {c.room}</span>
+                            <span>{t("schedule.room")} {c.room}</span>
                         </>
                     )}
                   </div>