fix(dashboard): v3 审计修复 — 数据完整性、i18n、类型安全、死代码清理

P0 修复（严重）:
- admin ContentRow 标签与值错配（stats.users→textbooks 等 6 处）
- admin/error.tsx 硬编码中文替换为 useTranslations
- UserGrowthChart 空数据时渲染 EmptyState（userGrowth/homeworkTrend 永远为空数组）

P1 修复（高）:
- 新增 admin/dashboard 和 student/dashboard 的 loading.tsx + error.tsx
- 抽取 DashboardLoadingSkeleton 和 DashboardErrorFallback 共享组件，消除 5 套重复文件
- formatDate/formatLongDate 传入用户 locale（admin/teacher/student 共 6 个组件）
- 移除死代码：getCachedAdminDashboard、AvatarImage src={undefined}、TeacherStats isLoading prop
- filterTodaySchedule 改为泛型函数，消除 as 类型断言
- 辅助函数 getStatus/getDueUrgency 新增显式返回类型
- UserGrowthChart 新增 labelKey prop 区分用户增长/作业提交趋势标签

P2 修复（中）:
- 4 个组件从客户端转为服务端组件（DashboardGreetingHeader、TeacherQuickActions、TeacherDashboardHeader、StudentDashboardHeader）
- Student dashboard 空状态新增 CTA（viewSchedule、viewAll）
- TeacherHomeworkCard 图标按钮新增 aria-label
- TeacherTodoCard 排序逻辑重写为可读的 if/return 模式

同步更新:
- docs/architecture/005_architecture_data.json 新增 DashboardLoadingSkeleton、DashboardErrorFallback 条目
- 新增 docs/architecture/audit/dashboard-audit-report-v3.md 审计报告
- dashboard.json 新增 6 个 i18n 键（textbooks/chapters/questions/exams/totalAssignments/totalSubmissions）

src/shared/i18n/messages/en/exam-homework.json

@@ -44,7 +44,8 @@
       "2": "Easy-Med",
       "3": "Medium",
       "4": "Med-Hard",
-      "5": "Hard"
+      "5": "Hard",
+      "ariaLabel": "Difficulty level {{level}}: {{label}}"
     },
     "actions": {
       "preview": "Preview Exam",
@@ -191,6 +192,8 @@
       "noDescription": "No description provided.",
       "progress": "Progress",
       "jumpToQuestion": "Jump to question {{index}}",
+      "answered": "Answered",
+      "unanswered": "Not answered",
       "yourAnswer": "Your answer",
       "answerPlaceholder": "Type your answer here...",
       "true": "True",
@@ -198,7 +201,13 @@
       "unsupportedType": "Unsupported question type",
       "teacherFeedback": "Teacher Feedback",
       "noFeedback": "No specific feedback provided.",
-      "makeSureAnswered": "Make sure you have answered all questions."
+      "makeSureAnswered": "Make sure you have answered all questions.",
+      "autoSaveIdle": "Auto-save ready",
+      "autoSaveSaving": "Auto-saving...",
+      "autoSaveSaved": "Auto-saved",
+      "autoSaveError": "Auto-save failed. Will retry when network recovers.",
+      "autoSaveRestored": "Restored unsaved answers from offline cache",
+      "autoSaveCacheError": "Failed to restore offline cache"
     },
     "grade": {
       "title": "Grade",
@@ -248,7 +257,19 @@
       "correctAnswer": "Correct Answer",
       "teacherFeedback": "Teacher Feedback",
       "score": "Score",
-      "maxScore": "Max Score"
+      "maxScore": "Max Score",
+      "correctMarker": "✓ Correct",
+      "backToList": "Back to List",
+      "gradedReport": "Graded Report",
+      "submissionDetails": "Submission Details",
+      "questionsUnit": "Questions",
+      "noAnswer": "No answer provided",
+      "noFeedback": "No specific feedback provided.",
+      "questionBreakdown": "Question Breakdown",
+      "responseSummary": "Response Summary",
+      "description": "Description",
+      "noDescription": "No description provided.",
+      "totalScore": "Total Score"
     },
     "status": {
       "draft": "Draft",