feat(maintenance): show maintenance page when the API is in maintenance

The backend now answers 503 with code MAINTENANCE_MODE while maintenance is
on. Without this change a visitor got a red error toast over a broken page
client-side, and a silently empty page server-side, because fetchReq discards
the status and returns null on any failure.

- lib/maintenance.js detects the state by BOTH status 503 and the error code;
  a bare 503 can come from a reverse proxy and is not maintenance
- The axios interceptor checks it before the 401 branch, so a maintenance
  response never triggers the refresh-token path or logs the user out
- fetchReq redirects to /maintenance, with a silentMaintenance opt-out used by
  getStateInfo: that one runs inside generateMetadata and while rendering the
  maintenance page itself, where a redirect is either ineffective or loops
- redirect() works by throwing, so the try/catch blocks in the doctors,
  clinics and specialties pages now rethrow NEXT_REDIRECT instead of
  swallowing it
- clinicApi.js handles 503 too; it previously rendered maintenance as a clinic
  with zero doctors
- The page reuses the existing 404 design and is marked noindex

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

app/maintenance/page.js

@@ -0,0 +1,23 @@
+import Layout from "@/components/layout/StLayout";
+import MaintenancePage from "@/components/maintenance";
+
+// وضعیت تعمیرات هر لحظه ممکن است عوض شود؛ این صفحه هرگز نباید کش شود.
+export const dynamic = "force-dynamic";
+
+export async function generateMetadata() {
+  return {
+    title: "در حال به‌روزرسانی سیستم",
+    description: "سامانه موقتاً برای انجام عملیات فنی در دسترس نیست.",
+    robots: { index: false, follow: false },
+  };
+}
+
+function Maintenance() {
+  return (
+    <Layout title="در حال به‌روزرسانی سیستم" disableFooter={true}>
+      <MaintenancePage />
+    </Layout>
+  );
+}
+
+export default Maintenance;