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/modules/dashboard/components/student-dashboard/student-dashboard-view.tsx

@@ -1,5 +1,6 @@
 import type { StudentDashboardProps } from "@/modules/dashboard/types"
 
+import { DashboardSection } from "../dashboard-section"
 import { StudentDashboardHeader } from "./student-dashboard-header"
 import { StudentGradesCard } from "./student-grades-card"
 import { StudentStatsGrid } from "./student-stats-grid"
@@ -20,21 +21,29 @@ export async function StudentDashboard({
     <div className="space-y-6">
       <StudentDashboardHeader studentName={studentName} />
 
-      <StudentStatsGrid
-        enrolledClassCount={enrolledClassCount}
-        dueSoonCount={dueSoonCount}
-        overdueCount={overdueCount}
-        gradedCount={gradedCount}
-        ranking={grades.ranking}
-      />
+      <DashboardSection variant="stats">
+        <StudentStatsGrid
+          enrolledClassCount={enrolledClassCount}
+          dueSoonCount={dueSoonCount}
+          overdueCount={overdueCount}
+          gradedCount={gradedCount}
+          ranking={grades.ranking}
+        />
+      </DashboardSection>
 
       <div className="grid grid-cols-1 lg:grid-cols-3 gap-6">
         <div className="lg:col-span-2 space-y-6">
-          <StudentUpcomingAssignmentsCard upcomingAssignments={upcomingAssignments} />
-          <StudentGradesCard grades={grades} />
+          <DashboardSection variant="list">
+            <StudentUpcomingAssignmentsCard upcomingAssignments={upcomingAssignments} />
+          </DashboardSection>
+          <DashboardSection variant="card">
+            <StudentGradesCard grades={grades} />
+          </DashboardSection>
         </div>
         <div className="space-y-6">
-          <StudentTodayScheduleCard items={todayScheduleItems} />
+          <DashboardSection variant="card">
+            <StudentTodayScheduleCard items={todayScheduleItems} />
+          </DashboardSection>
         </div>
       </div>
     </div>