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/modules/dashboard/components/teacher-dashboard/teacher-dashboard-header.tsx

@@ -1,6 +1,8 @@
 "use client"
 
+import { useTranslations } from "next-intl"
 import { formatLongDate } from "@/shared/lib/utils"
+import { getGreetingKey } from "@/modules/dashboard/lib/dashboard-utils"
 import { TeacherQuickActions } from "./teacher-quick-actions"
 
 interface TeacherDashboardHeaderProps {
@@ -8,18 +10,17 @@ interface TeacherDashboardHeaderProps {
 }
 
 export function TeacherDashboardHeader({ teacherName }: TeacherDashboardHeaderProps) {
+  const t = useTranslations("dashboard")
   const today = formatLongDate(new Date())
-  const hour = new Date().getHours()
-  let greeting = "欢迎回来"
-  if (hour < 12) greeting = "早上好"
-  else if (hour < 18) greeting = "下午好"
-  else greeting = "晚上好"
+  const greetingKey = getGreetingKey(new Date())
 
   return (
     <div className="flex flex-col justify-between space-y-4 md:flex-row md:items-center md:space-y-0">
       <div>
-        <h2 className="text-2xl font-bold tracking-tight">{greeting}，{teacherName}</h2>
-        <p className="text-muted-foreground">今天是 {today}，以下是今日概览。</p>
+        <h2 className="text-2xl font-bold tracking-tight">
+          {t(`greeting.${greetingKey}`)}，{teacherName}
+        </h2>
+        <p className="text-muted-foreground">{t("greeting.todayIs", { date: today })}</p>
       </div>
       <TeacherQuickActions />
     </div>