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/teacher-dashboard/teacher-dashboard-view.tsx

@@ -3,6 +3,7 @@ import type { TeacherDashboardMetrics } from "@/modules/dashboard/lib/dashboard-
 import { getTranslations } from "next-intl/server"
 import type { TeacherTodoItem } from "./teacher-todo-card"
 
+import { DashboardSection } from "../dashboard-section"
 import { TeacherClassesCard } from "./teacher-classes-card"
 import { TeacherDashboardHeader } from "./teacher-dashboard-header"
 import { TeacherHomeworkCard } from "./teacher-homework-card"
@@ -46,35 +47,51 @@ export async function TeacherDashboardView({ data }: TeacherDashboardViewProps)
     <div className="flex h-full flex-col space-y-6 p-8">
       <TeacherDashboardHeader teacherName={data.teacherName} />
 
-      <TeacherStats
-        toGradeCount={metrics.toGradeCount}
-        activeAssignmentsCount={metrics.activeAssignmentsCount}
-        averageScore={metrics.averageScore}
-        submissionRate={metrics.submissionRate}
-      />
+      <DashboardSection variant="stats">
+        <TeacherStats
+          toGradeCount={metrics.toGradeCount}
+          activeAssignmentsCount={metrics.activeAssignmentsCount}
+          averageScore={metrics.averageScore}
+          submissionRate={metrics.submissionRate}
+        />
+      </DashboardSection>
 
       <div className="grid gap-6 lg:grid-cols-12">
         {/* 移动端优先展示：今日课表 → 待办 → 待批改 */}
         <div className="flex flex-col gap-6 lg:col-span-8">
           <div className="lg:hidden">
-            <TeacherSchedule items={metrics.todayScheduleItems} />
+            <DashboardSection variant="card">
+              <TeacherSchedule items={metrics.todayScheduleItems} />
+            </DashboardSection>
           </div>
-          <TeacherTodoCard items={todoItems} />
-          <TeacherGradeTrends trends={data.gradeTrends} />
-          <RecentSubmissions
-            submissions={metrics.submissionsToGrade}
-            title={t("sections.pendingGrading")}
-            emptyTitle={t("empty.allGraded")}
-            emptyDescription={t("empty.allGradedDesc")}
-          />
+          <DashboardSection variant="card">
+            <TeacherTodoCard items={todoItems} />
+          </DashboardSection>
+          <DashboardSection variant="chart">
+            <TeacherGradeTrends trends={data.gradeTrends} />
+          </DashboardSection>
+          <DashboardSection variant="list">
+            <RecentSubmissions
+              submissions={metrics.submissionsToGrade}
+              title={t("sections.pendingGrading")}
+              emptyTitle={t("empty.allGraded")}
+              emptyDescription={t("empty.allGradedDesc")}
+            />
+          </DashboardSection>
         </div>
 
         <div className="flex flex-col gap-6 lg:col-span-4">
           <div className="hidden lg:block">
-            <TeacherSchedule items={metrics.todayScheduleItems} />
+            <DashboardSection variant="card">
+              <TeacherSchedule items={metrics.todayScheduleItems} />
+            </DashboardSection>
           </div>
-          <TeacherHomeworkCard assignments={data.assignments} />
-          <TeacherClassesCard classes={data.classes} />
+          <DashboardSection variant="list">
+            <TeacherHomeworkCard assignments={data.assignments} />
+          </DashboardSection>
+          <DashboardSection variant="list">
+            <TeacherClassesCard classes={data.classes} />
+          </DashboardSection>
         </div>
       </div>
     </div>