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)/teacher/elective/error.tsx

@@ -0,0 +1,24 @@
+"use client"
+
+import { AlertCircle } from "lucide-react"
+import { useTranslations } from "next-intl"
+
+import { EmptyState } from "@/shared/components/ui/empty-state"
+
+export default function TeacherElectiveError({ reset }: { error: Error & { digest?: string }; reset: () => void }) {
+  const t = useTranslations("elective")
+  return (
+    <div className="flex h-full flex-col items-center justify-center space-y-4 p-8">
+      <EmptyState
+        icon={AlertCircle}
+        title={t("errors.unexpected")}
+        description={t("errors.unexpected")}
+        action={{
+          label: t("actions.save"),
+          onClick: () => reset(),
+        }}
+        className="border-none shadow-none h-auto"
+      />
+    </div>
+  )
+}