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/shared/i18n/messages/zh-CN/dashboard.json

@@ -64,7 +64,17 @@
     "grades": "成绩",
     "attendance": "考勤",
     "announcements": "通知",
-    "leaveRequest": "请假申请"
+    "leaveRequest": "请假申请",
+    "createAssignment": "新建作业",
+    "grade": "批改",
+    "myClasses": "我的班级",
+    "viewAll": "查看全部",
+    "viewSchedule": "查看课表",
+    "viewAllAssignments": "查看全部作业",
+    "viewAllSubmissions": "查看全部提交",
+    "createNewAssignment": "新建作业",
+    "create": "新建",
+    "createClass": "新建班级"
   },
   "todo": {
     "title": "今日待办",
@@ -86,13 +96,26 @@
     "upcomingAssignments": "即将到期的作业",
     "grades": "成绩",
     "myClasses": "我的班级",
-    "gradeTrends": "成绩趋势"
+    "gradeTrends": "成绩趋势",
+    "homework": "作业",
+    "recentSubmissions": "最近提交",
+    "classPerformance": "班级表现",
+    "recentGrades": "最近成绩"
   },
   "table": {
     "name": "姓名",
     "email": "邮箱",
     "role": "角色",
-    "created": "创建时间"
+    "created": "创建时间",
+    "student": "学生",
+    "assignment": "作业",
+    "submitted": "提交时间",
+    "action": "操作",
+    "title": "标题",
+    "status": "状态",
+    "due": "截止时间",
+    "score": "分数",
+    "when": "时间"
   },
   "empty": {
     "noUsers": "暂无用户",
@@ -105,12 +128,31 @@
     "noChildrenDesc": "您的账号尚未关联任何学生账号，请联系学校管理员完成绑定。",
     "noStudent": "未找到学生用户",
     "noStudentDesc": "请创建学生账号以查看仪表盘。",
-    "contactSupport": "联系客服"
+    "contactSupport": "联系客服",
+    "noClassesYet": "暂无班级",
+    "noClassesDesc": "创建班级以开始管理学生与课表。",
+    "noAssignments": "暂无作业",
+    "noAssignmentsDesc": "创建作业以开始布置任务。",
+    "noAssignmentsStudent": "暂无作业",
+    "noAssignmentsStudentDesc": "当前没有布置给你的作业。",
+    "noClassesToday": "今日无课",
+    "noClassesTodayDesc": "今日课表为空。",
+    "noNewSubmissions": "暂无新提交",
+    "noNewSubmissionsDesc": "全部处理完成！暂无新提交需要批改。",
+    "noData": "暂无数据",
+    "noDataDesc": "发布作业后即可查看班级表现趋势。",
+    "noGradedWork": "暂无已批改作业",
+    "noGradedWorkDesc": "完成并提交作业后即可查看成绩趋势。"
   },
   "badge": {
     "activeSessions": "{count} 个活跃会话",
     "users": "{count} 位用户",
-    "childrenLinked": "已关联 {count} 个孩子"
+    "childrenLinked": "已关联 {count} 个孩子",
+    "live": "进行中",
+    "late": "已迟交",
+    "inProgress": "进行中",
+    "upNext": "下一节",
+    "unknown": "未知"
   },
   "error": {
     "loadFailed": "页面加载失败",
@@ -121,6 +163,26 @@
   },
   "chart": {
     "newUsers": "新增用户",
-    "newSubmissions": "新增提交"
+    "newSubmissions": "新增提交",
+    "averageScorePercent": "平均分 (%)",
+    "scorePercent": "分数 (%)",
+    "classPerformanceDesc": "最近 {count} 份作业的平均分",
+    "submittedCount": "{submitted}/{total} 已提交",
+    "latest": "最新",
+    "points": "得分"
+  },
+  "schedule": {
+    "noDueDate": "无截止日期",
+    "scrollForMore": "滚动查看更多",
+    "noMoreClasses": "今日无更多课程",
+    "homeroom": "班主任",
+    "room": "教室"
+  },
+  "action": {
+    "review": "查看",
+    "view": "查看",
+    "continue": "继续",
+    "start": "开始",
+    "grade": "批改"
   }
 }