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/shared/lib/permissions.ts

@@ -62,6 +62,7 @@ export const ROLE_PERMISSIONS: Record<Role, Permission[]> = {
     Permissions.FILE_UPLOAD,
     Permissions.FILE_READ,
     Permissions.FILE_DELETE,
+    Permissions.DASHBOARD_ADMIN_READ,
   ],
   teacher: [
     Permissions.EXAM_CREATE,
@@ -105,6 +106,7 @@ export const ROLE_PERMISSIONS: Record<Role, Permission[]> = {
     Permissions.LESSON_PLAN_UPDATE,
     Permissions.LESSON_PLAN_DELETE,
     Permissions.LESSON_PLAN_PUBLISH,
+    Permissions.DASHBOARD_TEACHER_READ,
   ],
   student: [
     Permissions.EXAM_READ,
@@ -125,6 +127,7 @@ export const ROLE_PERMISSIONS: Record<Role, Permission[]> = {
     Permissions.ELECTIVE_SELECT,
     Permissions.ELECTIVE_READ,
     Permissions.DIAGNOSTIC_READ,
+    Permissions.DASHBOARD_STUDENT_READ,
   ],
   parent: [
     Permissions.EXAM_READ,
@@ -137,6 +140,7 @@ export const ROLE_PERMISSIONS: Record<Role, Permission[]> = {
     Permissions.MESSAGE_SEND,
     Permissions.MESSAGE_READ,
     Permissions.MESSAGE_DELETE,
+    Permissions.DASHBOARD_PARENT_READ,
   ],
   grade_head: [
     Permissions.EXAM_CREATE,