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-grade-trends.tsx

@@ -1,12 +1,14 @@
 "use client"
 
 import { TrendingUp } from "lucide-react"
+import { useTranslations } from "next-intl"
 
 import { ChartCardShell } from "@/shared/components/charts/chart-card-shell"
 import { TrendLineChart } from "@/shared/components/charts/trend-line-chart"
 import type { TeacherGradeTrendItem } from "@/modules/homework/types"
 
 export function TeacherGradeTrends({ trends }: { trends: TeacherGradeTrendItem[] }) {
+  const t = useTranslations("dashboard")
   const hasTrends = trends.length > 0
 
   const chartData = trends.map((item) => {
@@ -22,14 +24,14 @@ export function TeacherGradeTrends({ trends }: { trends: TeacherGradeTrendItem[]
 
   return (
     <ChartCardShell
-      title="Class Performance"
-      description={`Average scores for the last ${trends.length} assignments`}
+      title={t("sections.classPerformance")}
+      description={t("chart.classPerformanceDesc", { count: trends.length })}
       icon={TrendingUp}
       iconClassName="text-primary"
       titleClassName="text-base font-medium"
       isEmpty={!hasTrends}
-      emptyTitle="No data available"
-      emptyDescription="Publish assignments to see class performance trends."
+      emptyTitle={t("empty.noData")}
+      emptyDescription={t("empty.noDataDesc")}
       emptyClassName="h-[200px] p-0"
       className="col-span-1"
     >
@@ -39,7 +41,7 @@ export function TeacherGradeTrends({ trends }: { trends: TeacherGradeTrendItem[]
           series={[
             {
               dataKey: "score",
-              name: "Average Score (%)",
+              name: t("chart.averageScorePercent"),
               color: "hsl(var(--primary))",
               dotRadius: 4,
               activeDotRadius: 6,
@@ -65,7 +67,7 @@ export function TeacherGradeTrends({ trends }: { trends: TeacherGradeTrendItem[]
                   <span className="text-xl font-bold tabular-nums">{item.score}%</span>
                 </div>
                 <div className="text-[10px] text-muted-foreground">
-                  {item.submissionCount}/{item.totalStudents} submitted
+                  {t("chart.submittedCount", { submitted: item.submissionCount, total: item.totalStudents })}
                 </div>
               </div>
             ))}