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/student-dashboard/student-dashboard-header.tsx

@@ -1,26 +1,13 @@
 "use client"
 
-import { useTranslations } from "next-intl"
-import { formatLongDate } from "@/shared/lib/utils"
-import { getGreetingKey } from "@/modules/dashboard/lib/dashboard-utils"
+import { DashboardGreetingHeader } from "../dashboard-greeting-header"
 
 interface StudentDashboardHeaderProps {
   studentName: string
 }
 
 export function StudentDashboardHeader({ studentName }: StudentDashboardHeaderProps) {
-  const t = useTranslations("dashboard")
-  const today = formatLongDate(new Date())
-  const greetingKey = getGreetingKey(new Date())
-
   return (
-    <div className="flex flex-col justify-between space-y-4 md:flex-row md:items-center md:space-y-0">
-      <div>
-        <h2 className="text-2xl font-bold tracking-tight">
-          {t(`greeting.${greetingKey}`)}，{studentName}
-        </h2>
-        <p className="text-muted-foreground">{t("greeting.todayIs", { date: today })}</p>
-      </div>
-    </div>
+    <DashboardGreetingHeader userName={studentName} />
   )
 }