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-view.tsx

@@ -1,3 +1,5 @@
+import { getTranslations } from "next-intl/server"
+
 import type { StudentDashboardProps } from "@/modules/dashboard/types"
 
 import { DashboardSection } from "../dashboard-section"
@@ -17,9 +19,12 @@ export async function StudentDashboard({
   upcomingAssignments,
   grades,
 }: StudentDashboardProps) {
+  const t = await getTranslations("dashboard")
   return (
     <div className="space-y-6">
-      <StudentDashboardHeader studentName={studentName} />
+      <header>
+        <StudentDashboardHeader studentName={studentName} />
+      </header>
 
       <DashboardSection variant="stats">
         <StudentStatsGrid
@@ -32,19 +37,22 @@ export async function StudentDashboard({
       </DashboardSection>
 
       <div className="grid grid-cols-1 lg:grid-cols-3 gap-6">
-        <div className="lg:col-span-2 space-y-6">
+        <section
+          aria-label={t("sections.upcomingAssignments")}
+          className="lg:col-span-2 space-y-6"
+        >
           <DashboardSection variant="list">
             <StudentUpcomingAssignmentsCard upcomingAssignments={upcomingAssignments} />
           </DashboardSection>
           <DashboardSection variant="card">
             <StudentGradesCard grades={grades} />
           </DashboardSection>
-        </div>
-        <div className="space-y-6">
+        </section>
+        <aside aria-label={t("sections.todaySchedule")} className="space-y-6">
           <DashboardSection variant="card">
             <StudentTodayScheduleCard items={todayScheduleItems} />
           </DashboardSection>
-        </div>
+        </aside>
       </div>
     </div>
   )