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)/dashboard/page.tsx

@@ -1,16 +1,28 @@
 import { redirect } from "next/navigation"
+
 import { auth } from "@/auth"
+import { Permissions } from "@/shared/types/permissions"
+import { resolvePermissions } from "@/shared/lib/permissions"
+import { isRole } from "@/shared/types/permissions"
 
 export const dynamic = "force-dynamic"
 
-export default async function DashboardPage() {
+export default async function DashboardPage(): Promise<void> {
   const session = await auth()
   if (!session?.user) redirect("/login")
 
-  const roles = session.user.roles ?? []
+  const roles = (session.user.roles ?? []).filter(isRole)
+  const permissions = resolvePermissions(roles)
 
-  if (roles.includes("admin")) redirect("/admin/dashboard")
-  if (roles.includes("student")) redirect("/student/dashboard")
-  if (roles.includes("parent")) redirect("/parent/dashboard")
+  // 按优先级匹配仪表盘权限（admin > student > parent > teacher）
+  if (permissions.includes(Permissions.DASHBOARD_ADMIN_READ)) {
+    redirect("/admin/dashboard")
+  }
+  if (permissions.includes(Permissions.DASHBOARD_STUDENT_READ)) {
+    redirect("/student/dashboard")
+  }
+  if (permissions.includes(Permissions.DASHBOARD_PARENT_READ)) {
+    redirect("/parent/dashboard")
+  }
   redirect("/teacher/dashboard")
 }