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/actions.ts

@@ -18,6 +18,7 @@ import {
   persistExamDraft,
   resolveSubjectGradeNames,
   updateExamWithQuestions,
+  type ExamModeConfig,
 } from "./data-access"
 import {
   AiGeneratedStructureSchema,
@@ -58,6 +59,30 @@ const getStringValue = (formData: FormData, key: string) => {
   return typeof value === "string" ? value : undefined
 }
 
+const getBoolValue = (formData: FormData, key: string, fallback = false): boolean => {
+  const value = formData.get(key)
+  if (typeof value !== "string") return fallback
+  return value === "true"
+}
+
+const parseExamModeConfig = (formData: FormData): ExamModeConfig => {
+  const rawMode = getStringValue(formData, "examMode")
+  const examMode: ExamModeConfig["examMode"] =
+    rawMode === "timed" || rawMode === "proctored" ? rawMode : "homework"
+  const rawDuration = getStringValue(formData, "durationMinutes")
+  const durationMinutes = rawDuration && Number.isFinite(Number(rawDuration))
+    ? Number(rawDuration)
+    : null
+  return {
+    examMode,
+    durationMinutes,
+    shuffleQuestions: getBoolValue(formData, "shuffleQuestions", false),
+    allowLateStart: getBoolValue(formData, "allowLateStart", false),
+    lateStartGraceMinutes: Number(getStringValue(formData, "lateStartGraceMinutes") ?? "0") || 0,
+    antiCheatEnabled: getBoolValue(formData, "antiCheatEnabled", false),
+  }
+}
+
 const failState = <T>(message: string, errors?: Record<string, string[]>): ActionState<T> => ({
   success: false,
   message,
@@ -317,6 +342,7 @@ export async function createExamAction(
         gradeId: input.grade,
         scheduledAt: context.scheduled,
         description,
+        examModeConfig: parseExamModeConfig(formData),
       })
     } catch (error) {
       console.error("Failed to create exam:", error)
@@ -436,6 +462,7 @@ export async function createAiExamAction(
         description,
         structure,
         generated,
+        examModeConfig: parseExamModeConfig(formData),
       })
     } catch (error) {
       console.error("Failed to create exam:", error)