feat(dashboard): 新增分区 Error Boundary + Suspense 骨架屏（P2）

新增 components/dashboard-section.tsx，包含：

- DashboardSectionErrorBoundary：分区级 Error Boundary，单区块崩溃仅替换该区块不波及整页

- DashboardSectionSkeleton：5 种骨架变体（stats/card/chart/table/list），匹配不同数据区块布局

- DashboardSection：组合 Error Boundary + Suspense + 骨架屏的包装器

将 admin/teacher/student 三个仪表盘视图的每个独立数据区块用 DashboardSection 包裹，i18n 补充 sectionLoadFailed/sectionLoadFailedDesc 翻译键，同步更新架构图 004/005 文档

src/shared/i18n/messages/en/dashboard.json

@@ -115,7 +115,9 @@
   "error": {
     "loadFailed": "Page load failed",
     "loadFailedDesc": "Sorry, an unexpected error occurred while loading the page. Please try again later.",
-    "retry": "Retry"
+    "retry": "Retry",
+    "sectionLoadFailed": "Section load failed",
+    "sectionLoadFailedDesc": "An error occurred while loading this section. Please retry."
   },
   "chart": {
     "newUsers": "New users",