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/modules/exams/components/exam-actions.tsx

@@ -36,7 +36,27 @@ import { deleteExamAction, duplicateExamAction, updateExamAction, getExamPreview
 import { Exam } from "../types"
 import { ExamPaperPreview } from "./assembly/exam-paper-preview"
 import type { ExamNode } from "./assembly/selected-question-list"
-import type { Question } from "@/modules/questions/types"
+
+// Raw structure node shape returned from the DB before hydration
+type RawStructureNode = {
+  id?: string
+  type?: string
+  questionId?: string
+  score?: number
+  title?: string
+  children?: RawStructureNode[]
+}
+
+// Type guard to narrow unknown structure payload to raw nodes
+const isRawStructureNode = (v: unknown): v is RawStructureNode => {
+  if (typeof v !== "object" || v === null) return false
+  // 从 unknown 收窄为 Record<string, unknown> 以进行字段检查
+  const obj = v as Record<string, unknown>
+  return typeof obj.type === "string"
+}
+
+const isRawStructureArray = (v: unknown): v is RawStructureNode[] =>
+  Array.isArray(v) && v.every((item) => isRawStructureNode(item))
 
 interface ExamActionsProps {
   exam: Exam
@@ -57,25 +77,39 @@ export function ExamActions({ exam }: ExamActionsProps) {
     try {
       const result = await getExamPreviewAction(exam.id)
       if (result.success && result.data) {
-        const { structure, questions } = result.data
-        const questionById = new Map<string, Question>()
-        for (const q of questions) questionById.set(q.id, q as unknown as Question)
+        const { structure } = result.data
 
-        // eslint-disable-next-line @typescript-eslint/no-explicit-any
-        const hydrate = (nodes: any[]): ExamNode[] => {
-            return nodes.map((node) => {
-                if (node.type === "question") {
-                    const q = node.questionId ? questionById.get(node.questionId) : undefined
-                    return { ...node, question: q }
-                }
-                if (node.type === "group") {
-                    return { ...node, children: hydrate(node.children || []) }
-                }
-                return node
-            })
+        const hydrate = (nodes: RawStructureNode[]): ExamNode[] => {
+          return nodes.map((node) => {
+            if (node.type === "question") {
+              return {
+                id: node.id ?? node.questionId ?? "",
+                type: "question" as const,
+                questionId: node.questionId,
+                score: node.score,
+                // Question content is not available in preview payload; left undefined
+              }
+            }
+            if (node.type === "group") {
+              return {
+                id: node.id ?? "",
+                type: "group" as const,
+                title: node.title,
+                score: node.score,
+                children: hydrate(node.children ?? []),
+              }
+            }
+            // Unknown node type: treat as group with no children to avoid runtime crash
+            return {
+              id: node.id ?? "",
+              type: "group" as const,
+              title: node.title,
+              children: [],
+            }
+          })
         }
-        
-        const nodes = Array.isArray(structure) ? hydrate(structure) : []
+
+        const nodes = isRawStructureArray(structure) ? hydrate(structure) : []
         setPreviewNodes(nodes)
       } else {
         toast.error(t("exam.actions.previewFailed"))