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/app/(dashboard)/management/grade/classes/loading.tsx

@@ -0,0 +1,24 @@
+import { Card, CardContent, CardHeader } from "@/shared/components/ui/card"
+import { Skeleton } from "@/shared/components/ui/skeleton"
+
+export default function ManagementGradeClassesLoading() {
+  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>
+
+      <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>
+  )
+}