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/lesson-preparation/services/default-data-service.ts

@@ -0,0 +1,50 @@
+"use client";
+
+import {
+  getLessonPlansAction,
+  getLessonPlanVersionsAction,
+  revertLessonPlanVersionAction,
+  duplicateLessonPlanAction,
+  deleteLessonPlanAction,
+} from "../actions";
+import type { LessonPlanDataService } from "../providers/lesson-plan-provider";
+
+/**
+ * 默认数据服务实现：包装现有 Server Actions。
+ * 通过 LessonPlanProvider 注入，组件不直接 import actions。
+ * 测试时可替换为 mock 实现。
+ */
+export function createDefaultDataService(): LessonPlanDataService {
+  return {
+    async getLessonPlans(params) {
+      const res = await getLessonPlansAction(params ?? {});
+      if (res.success && res.data) {
+        return { success: true, data: { items: res.data.items } };
+      }
+      return { success: false, message: res.message };
+    },
+
+    async getLessonPlanVersions(planId) {
+      const res = await getLessonPlanVersionsAction(planId);
+      if (res.success && res.data) {
+        return { success: true, data: { versions: res.data.versions } };
+      }
+      return { success: false, message: res.message };
+    },
+
+    async revertLessonPlanVersion(params) {
+      const res = await revertLessonPlanVersionAction(params);
+      return { success: res.success, message: res.message };
+    },
+
+    async duplicateLessonPlan(planId) {
+      const res = await duplicateLessonPlanAction(planId);
+      return { success: res.success, message: res.message };
+    },
+
+    async deleteLessonPlan(planId) {
+      const res = await deleteLessonPlanAction(planId);
+      return { success: res.success, message: res.message };
+    },
+  };
+}