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/dashboard-section.tsx

@@ -0,0 +1,170 @@
+"use client"
+
+import { Component, type ReactNode, Suspense } from "react"
+import { AlertCircle } from "lucide-react"
+
+import { EmptyState } from "@/shared/components/ui/empty-state"
+import { Card, CardContent, CardHeader } from "@/shared/components/ui/card"
+import { Skeleton } from "@/shared/components/ui/skeleton"
+import { useTranslations } from "next-intl"
+
+/**
+ * 仪表盘分区 Error Boundary
+ *
+ * 包裹每个独立数据区块，避免单个区块崩溃导致整页不可用。
+ * 与路由级 error.tsx 不同，此组件仅替换出错区块，其余区块继续渲染。
+ */
+export class DashboardSectionErrorBoundary extends Component<
+  { children: ReactNode },
+  { hasError: boolean }
+> {
+  state: { hasError: boolean } = { hasError: false }
+
+  static getDerivedStateFromError(): { hasError: boolean } {
+    return { hasError: true }
+  }
+
+  handleRetry = (): void => {
+    this.setState({ hasError: false })
+  }
+
+  render(): ReactNode {
+    if (this.state.hasError) {
+      return <DashboardSectionErrorFallback onRetry={this.handleRetry} />
+    }
+    return this.props.children
+  }
+}
+
+function DashboardSectionErrorFallback({
+  onRetry,
+}: {
+  onRetry: () => void
+}): ReactNode {
+  const t = useTranslations("dashboard.error")
+  return (
+    <EmptyState
+      icon={AlertCircle}
+      title={t("sectionLoadFailed")}
+      description={t("sectionLoadFailedDesc")}
+      action={{ label: t("retry"), onClick: onRetry }}
+      className="h-auto border-none shadow-none"
+    />
+  )
+}
+
+/**
+ * 分区骨架屏变体
+ *
+ * 不同数据区块使用不同骨架布局，提供更贴近真实内容的加载占位。
+ */
+type SkeletonVariant = "stats" | "card" | "chart" | "table" | "list"
+
+export function DashboardSectionSkeleton({
+  variant = "card",
+}: {
+  variant?: SkeletonVariant
+}): ReactNode {
+  switch (variant) {
+    case "stats":
+      return (
+        <div className="grid gap-4 md:grid-cols-2 lg:grid-cols-4">
+          {Array.from({ length: 4 }).map((_, i) => (
+            <Card key={i}>
+              <CardHeader className="pb-2">
+                <Skeleton className="h-4 w-24" />
+              </CardHeader>
+              <CardContent>
+                <Skeleton className="h-8 w-16" />
+                <Skeleton className="mt-2 h-3 w-28" />
+              </CardContent>
+            </Card>
+          ))}
+        </div>
+      )
+    case "chart":
+      return (
+        <Card>
+          <CardHeader>
+            <Skeleton className="h-5 w-32" />
+          </CardHeader>
+          <CardContent>
+            <Skeleton className="h-[200px] w-full" />
+          </CardContent>
+        </Card>
+      )
+    case "table":
+      return (
+        <Card>
+          <CardHeader>
+            <Skeleton className="h-5 w-32" />
+          </CardHeader>
+          <CardContent className="space-y-3">
+            {Array.from({ length: 5 }).map((_, i) => (
+              <Skeleton key={i} className="h-10 w-full" />
+            ))}
+          </CardContent>
+        </Card>
+      )
+    case "list":
+      return (
+        <Card>
+          <CardHeader>
+            <Skeleton className="h-5 w-32" />
+          </CardHeader>
+          <CardContent className="space-y-3">
+            {Array.from({ length: 4 }).map((_, i) => (
+              <div key={i} className="flex items-center gap-3">
+                <Skeleton className="h-9 w-9 rounded-full" />
+                <div className="flex-1 space-y-1.5">
+                  <Skeleton className="h-3 w-3/4" />
+                  <Skeleton className="h-3 w-1/2" />
+                </div>
+              </div>
+            ))}
+          </CardContent>
+        </Card>
+      )
+    case "card":
+    default:
+      return (
+        <Card>
+          <CardHeader>
+            <Skeleton className="h-5 w-32" />
+          </CardHeader>
+          <CardContent className="space-y-3">
+            <Skeleton className="h-4 w-full" />
+            <Skeleton className="h-4 w-5/6" />
+            <Skeleton className="h-4 w-4/6" />
+          </CardContent>
+        </Card>
+      )
+  }
+}
+
+/**
+ * 仪表盘分区包装器
+ *
+ * 组合 Error Boundary + Suspense + 骨架屏，包裹每个独立数据区块。
+ * 单个区块出错或加载中时，仅影响该区块，不波及整页。
+ *
+ * @example
+ * <DashboardSection variant="stats">
+ *   <TeacherStats ... />
+ * </DashboardSection>
+ */
+export function DashboardSection({
+  children,
+  variant = "card",
+}: {
+  children: ReactNode
+  variant?: SkeletonVariant
+}): ReactNode {
+  return (
+    <DashboardSectionErrorBoundary>
+      <Suspense fallback={<DashboardSectionSkeleton variant={variant} />}>
+        {children}
+      </Suspense>
+    </DashboardSectionErrorBoundary>
+  )
+}