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/clinics/page.js

@@ -1,4 +1,5 @@
 import { fetchReq } from "@/lib/req";
+import { isNextRedirectError } from "@/lib/maintenance";
 import ClinicsPage from "@/components/clinics";
 import Layout from "@/components/layout/StLayout";
 import { getStateInfo } from "@/lib/getStateInfo";
@@ -62,7 +63,10 @@ export default async function Clinics({ searchParams }) {
     clinics = await fetchReq(`${API_URL}/api/v1/clinics`, {
       params,
     });
-  } catch (err) { }
+  } catch (err) {
+    // ریدایرکت نکست با throw کار می‌کند؛ بدون این rethrow، ریدایرکت حالت تعمیرات بلعیده می‌شود.
+    if (isNextRedirectError(err)) throw err;
+  }
 
   return (
     <Layout name="/clinics">