feat(dashboard): 仪表盘模块审计重构 — 权限校验 + i18n + 逻辑抽离

基于 dashboard-audit-report.md 审计结论，对仪表盘模块进行 P0/P1 级修复：

- 新增 4 个 dashboard 权限点（DASHBOARD_ADMIN/TEACHER/STUDENT/PARENT_READ），补充到 permissions.ts 和角色-权限映射

- 新建 actions.ts：4 个 Server Action 均调用 requirePermission() 校验权限，消除 admin 页面零鉴权、teacher/student/parent 仅 requireAuth 的安全隐患

- 根重定向页 /dashboard 改用 resolvePermissions() + 权限点判断，不再 role === xxx 硬编码

- 新建 lib/dashboard-utils.ts：抽取 toWeekday / countStudentAssignments / sortUpcomingAssignments / filterTodaySchedule / computeTeacherMetrics / getGreetingKey 纯函数，与 UI 分离，便于单测

- 新建 messages/{zh-CN,en}/dashboard.json 翻译文件，i18n request.ts 加载 dashboard 命名空间；所有视图组件接入 useTranslations / getTranslations，消除中英混杂硬编码

- 重构 4 个角色 page.tsx：通过 actions 获取数据，generateMetadata 使用 i18n

- 同步更新架构图 004 / 005 文档（dashboard exports / permissions / 文件清单）

src/app/(dashboard)/admin/dashboard/page.tsx

@@ -1,17 +1,21 @@
-import type { Metadata } from "next"
+import type { Metadata } from "next"
 import type { JSX } from "react"
+import { getTranslations } from "next-intl/server"
 
 import { AdminDashboardView } from "@/modules/dashboard/components/admin-dashboard/admin-dashboard"
-import { getAdminDashboardData } from "@/modules/dashboard/data-access"
-
-export const metadata: Metadata = {
-  title: "管理控制台 - Next_Edu",
-  description: "系统管理总览",
-}
+import { getAdminDashboardAction } from "@/modules/dashboard/actions"
 
 export const dynamic = "force-dynamic"
 
+export async function generateMetadata(): Promise<Metadata> {
+  const t = await getTranslations("dashboard")
+  return {
+    title: t("title.admin"),
+    description: t("description.admin"),
+  }
+}
+
 export default async function AdminDashboardPage(): Promise<JSX.Element> {
-  const data = await getAdminDashboardData()
+  const data = await getAdminDashboardAction()
   return <AdminDashboardView data={data} />
 }