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/homework/hooks/use-debounced-auto-save.ts

@@ -0,0 +1,189 @@
+"use client"
+
+import { useCallback, useEffect, useRef, useState } from "react"
+import { saveHomeworkAnswerAction } from "../actions"
+
+type AutoSaveStatus = "idle" | "saving" | "saved" | "error"
+
+type AnswerMap = Record<string, { answer: unknown } | undefined>
+
+type UseDebouncedAutoSaveOptions = {
+  submissionId: string | null
+  answers: AnswerMap
+  enabled: boolean
+  debounceMs?: number
+  storageKey?: string
+}
+
+type UseDebouncedAutoSaveResult = {
+  status: AutoSaveStatus
+  lastSavedAt: number | null
+  flush: () => Promise<void>
+}
+
+/**
+ * P2-9: 学生答案自动保存 + 离线缓存
+ *
+ * - 答案变更后 debounce（默认 3 秒）自动保存到服务端
+ * - 同时写入 localStorage 作为离线缓存
+ * - 网络异常时标记 error，恢复后自动重试
+ * - 组件卸载时 flush 未保存的答案
+ */
+export function useDebouncedAutoSave({
+  submissionId,
+  answers,
+  enabled,
+  debounceMs = 3000,
+  storageKey,
+}: UseDebouncedAutoSaveOptions): UseDebouncedAutoSaveResult {
+  const [status, setStatus] = useState<AutoSaveStatus>("idle")
+  const [lastSavedAt, setLastSavedAt] = useState<number | null>(null)
+  const timerRef = useRef<ReturnType<typeof setTimeout> | null>(null)
+  const pendingRef = useRef<Map<string, unknown>>(new Map())
+  const savingRef = useRef(false)
+  const lastSavedAnswersRef = useRef<string>("")
+
+  // Persist to localStorage for offline recovery
+  const cacheToLocalStorage = useCallback(
+    (snapshot: AnswerMap) => {
+      if (!storageKey) return
+      try {
+        const serialized = JSON.stringify({
+          submissionId,
+          answers: snapshot,
+          timestamp: Date.now(),
+        })
+        window.localStorage.setItem(storageKey, serialized)
+      } catch {
+        // localStorage may be full or unavailable; silently ignore
+      }
+    },
+    [storageKey, submissionId]
+  )
+
+  // Save a batch of pending answers to the server
+  const savePending = useCallback(async () => {
+    if (!submissionId || savingRef.current) return
+    const pending = Array.from(pendingRef.current.entries())
+    if (pending.length === 0) return
+
+    savingRef.current = true
+    setStatus("saving")
+
+    let allOk = true
+    for (const [questionId, answer] of pending) {
+      const fd = new FormData()
+      fd.set("submissionId", submissionId)
+      fd.set("questionId", questionId)
+      fd.set("answerJson", JSON.stringify({ answer }))
+      const res = await saveHomeworkAnswerAction(null, fd)
+      if (!res.success) {
+        allOk = false
+      }
+    }
+
+    savingRef.current = false
+
+    if (allOk) {
+      pendingRef.current.clear()
+      setStatus("saved")
+      setLastSavedAt(Date.now())
+    } else {
+      setStatus("error")
+      // Keep pending items for retry on next change or manual flush
+    }
+  }, [submissionId])
+
+  // Schedule debounced save when answers change
+  useEffect(() => {
+    if (!enabled || !submissionId) return
+
+    const currentSnapshot = JSON.stringify(answers)
+    if (currentSnapshot === lastSavedAnswersRef.current) return
+
+    // Cache to localStorage immediately (offline safety net)
+    cacheToLocalStorage(answers)
+
+    // Collect changed question IDs
+    for (const questionId of Object.keys(answers)) {
+      const entry = answers[questionId]
+      if (entry !== undefined) {
+        pendingRef.current.set(questionId, entry.answer)
+      }
+    }
+
+    // Clear existing timer and set a new one
+    if (timerRef.current) {
+      clearTimeout(timerRef.current)
+    }
+    timerRef.current = setTimeout(() => {
+      void savePending()
+      lastSavedAnswersRef.current = currentSnapshot
+    }, debounceMs)
+
+    return () => {
+      if (timerRef.current) {
+        clearTimeout(timerRef.current)
+        timerRef.current = null
+      }
+    }
+  }, [answers, enabled, submissionId, debounceMs, cacheToLocalStorage, savePending])
+
+  // Flush on unmount
+  useEffect(() => {
+    return () => {
+      if (timerRef.current) {
+        clearTimeout(timerRef.current)
+      }
+      // Fire-and-forget final save
+      void savePending()
+    }
+  }, [savePending])
+
+  // Retry on window focus (network may have recovered)
+  useEffect(() => {
+    if (status !== "error") return
+    const handleFocus = () => {
+      void savePending()
+    }
+    window.addEventListener("focus", handleFocus)
+    return () => window.removeEventListener("focus", handleFocus)
+  }, [status, savePending])
+
+  const flush = useCallback(async () => {
+    if (timerRef.current) {
+      clearTimeout(timerRef.current)
+      timerRef.current = null
+    }
+    await savePending()
+    lastSavedAnswersRef.current = JSON.stringify(answers)
+  }, [answers, savePending])
+
+  return { status, lastSavedAt, flush }
+}
+
+/**
+ * 从 localStorage 恢复离线缓存的答案
+ */
+export function loadOfflineCache(storageKey: string): AnswerMap | null {
+  try {
+    const raw = window.localStorage.getItem(storageKey)
+    if (!raw) return null
+    const parsed = JSON.parse(raw) as { answers?: AnswerMap }
+    if (!parsed.answers || typeof parsed.answers !== "object") return null
+    return parsed.answers
+  } catch {
+    return null
+  }
+}
+
+/**
+ * 清除 localStorage 中的离线缓存
+ */
+export function clearOfflineCache(storageKey: string): void {
+  try {
+    window.localStorage.removeItem(storageKey)
+  } catch {
+    // ignore
+  }
+}