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/components/blocks/exercise-block.tsx

@@ -84,8 +84,8 @@ export function ExerciseBlock({ blockId, data, classes, textbookId, chapterId }:
                   : t("questionBank.inlineQuestion")}
               </span>
               <span className="text-xs">{t("questionBank.score", { score: item.score })}</span>
-              <button onClick={() => removeItem(idx)}>
-                <Trash2 className="w-3 h-3 text-error" />
+              <button onClick={() => removeItem(idx)} aria-label={t("action.delete")}>
+                <Trash2 className="w-3 h-3 text-error" aria-hidden="true" />
               </button>
             </div>
           ))}
@@ -97,7 +97,7 @@ export function ExerciseBlock({ blockId, data, classes, textbookId, chapterId }:
           size="sm"
           onClick={() => setShowBank(true)}
         >
-          <Plus className="w-3 h-3 mr-1" />
+          <Plus className="w-3 h-3 mr-1" aria-hidden="true" />
           {t("questionBank.fromBank")}
         </Button>
         <Button
@@ -105,7 +105,7 @@ export function ExerciseBlock({ blockId, data, classes, textbookId, chapterId }:
           size="sm"
           onClick={() => setShowInline(true)}
         >
-          <Plus className="w-3 h-3 mr-1" />
+          <Plus className="w-3 h-3 mr-1" aria-hidden="true" />
           {t("questionBank.inlineNew")}
         </Button>
         {data.publishedAssignmentId ? (