feat(app): add error/loading boundaries and update dashboard routes

- Add error.tsx and loading.tsx boundaries for admin, parent, student, teacher routes

- Add dashboard-error-fallback and dashboard-loading-skeleton components

- Add student/learning page, parent/leave routes, teacher textbook components

- Update existing app routes across auth, dashboard, and API endpoints

- Update proxy middleware and next-auth type declarations

src/modules/dashboard/components/dashboard-loading-skeleton.tsx

@@ -0,0 +1,44 @@
+import { Card, CardContent, CardHeader } from "@/shared/components/ui/card"
+import { Skeleton } from "@/shared/components/ui/skeleton"
+
+/**
+ * 仪表盘通用加载骨架屏。
+ *
+ * 用于各角色 dashboard 路由的 `loading.tsx`，消除重复代码。
+ * 布局：页头 + 4 列统计卡片 + 列表骨架。
+ */
+export function DashboardLoadingSkeleton() {
+  return (
+    <div className="flex h-full flex-col space-y-8 p-8">
+      <div className="space-y-2">
+        <Skeleton className="h-8 w-48" />
+        <Skeleton className="h-4 w-64" />
+      </div>
+
+      <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>
+
+      <Card>
+        <CardHeader>
+          <Skeleton className="h-5 w-32" />
+        </CardHeader>
+        <CardContent className="space-y-3">
+          {Array.from({ length: 6 }).map((_, i) => (
+            <Skeleton key={i} className="h-12 w-full" />
+          ))}
+        </CardContent>
+      </Card>
+    </div>
+  )
+}