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-dashboard-view.tsx

@@ -45,7 +45,9 @@ export async function TeacherDashboardView({ data }: TeacherDashboardViewProps)
 
   return (
     <div className="flex h-full flex-col space-y-6 p-8">
-      <TeacherDashboardHeader teacherName={data.teacherName} />
+      <header>
+        <TeacherDashboardHeader teacherName={data.teacherName} />
+      </header>
 
       <DashboardSection variant="stats">
         <TeacherStats
@@ -57,8 +59,7 @@ export async function TeacherDashboardView({ data }: TeacherDashboardViewProps)
       </DashboardSection>
 
       <div className="grid gap-6 lg:grid-cols-12">
-        {/* 移动端优先展示：今日课表 → 待办 → 待批改 */}
-        <div className="flex flex-col gap-6 lg:col-span-8">
+        <section aria-label={t("sections.pendingGrading")} className="flex flex-col gap-6 lg:col-span-8">
           <div className="lg:hidden">
             <DashboardSection variant="card">
               <TeacherSchedule items={metrics.todayScheduleItems} />
@@ -78,9 +79,9 @@ export async function TeacherDashboardView({ data }: TeacherDashboardViewProps)
               emptyDescription={t("empty.allGradedDesc")}
             />
           </DashboardSection>
-        </div>
+        </section>
 
-        <div className="flex flex-col gap-6 lg:col-span-4">
+        <aside aria-label={t("sections.myClasses")} className="flex flex-col gap-6 lg:col-span-4">
           <div className="hidden lg:block">
             <DashboardSection variant="card">
               <TeacherSchedule items={metrics.todayScheduleItems} />
@@ -92,7 +93,7 @@ export async function TeacherDashboardView({ data }: TeacherDashboardViewProps)
           <DashboardSection variant="list">
             <TeacherClassesCard classes={data.classes} />
           </DashboardSection>
-        </div>
+        </aside>
       </div>
     </div>
   )