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/app/(dashboard)/teacher/homework/assignments/create/page.tsx

@@ -1,3 +1,4 @@
+import type { JSX } from "react"
 import { HomeworkAssignmentForm } from "@/modules/homework/components/homework-assignment-form"
 import { getExams } from "@/modules/exams/data-access"
 import { getTeacherClasses } from "@/modules/classes/data-access"
@@ -7,7 +8,7 @@ import { FileQuestion } from "lucide-react"
 
 export const dynamic = "force-dynamic"
 
-export default async function CreateHomeworkAssignmentPage() {
+export default async function CreateHomeworkAssignmentPage(): Promise<JSX.Element> {
   const { dataScope } = await getAuthContext()
   const [exams, classes] = await Promise.all([getExams({ scope: dataScope }), getTeacherClasses()])
   const options = exams.map((e) => ({ id: e.id, title: e.title }))
@@ -16,19 +17,12 @@ export default async function CreateHomeworkAssignmentPage() {
     <div className="flex h-full flex-col space-y-8 p-8">
       <div className="flex items-center justify-between">
         <div>
-          <h2 className="text-2xl font-bold tracking-tight">Create Assignment</h2>
-          <p className="text-muted-foreground">Dispatch homework from an existing exam.</p>
+          <h1 className="text-2xl font-bold tracking-tight">Create Assignment</h1>
+          <p className="text-muted-foreground">快速发布文本作业或从考试派生。</p>
         </div>
       </div>
 
-      {options.length === 0 ? (
-        <EmptyState
-          title="No exams available"
-          description="Create an exam first, then dispatch it as homework."
-          icon={FileQuestion}
-          action={{ label: "Create Exam", href: "/teacher/exams/create" }}
-        />
-      ) : classes.length === 0 ? (
+      {classes.length === 0 ? (
         <EmptyState
           title="No classes available"
           description="Create a class first, then publish homework to that class."