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>

lib/maintenance.js

@@ -0,0 +1,28 @@
+export const MAINTENANCE_CODE = "MAINTENANCE_MODE";
+
+export const MAINTENANCE_PATH = "/maintenance";
+
+export const DEFAULT_MAINTENANCE_MESSAGE =
+  "سامانه موقتاً در دسترس نیست. لطفاً چند دقیقه دیگر مجدداً تلاش کنید.";
+
+/**
+ * تشخیص پاسخ حالت تعمیرات backend.
+ *
+ * هر دو شرط لازم است: یک ۵۰۳ خالی می‌تواند از reverse proxy یا load balancer هم
+ * بیاید و آن حالت تعمیرات نیست.
+ */
+export function isMaintenanceError(error) {
+  const response = error?.response;
+  if (response?.status !== 503) return false;
+
+  return response?.data?.errors?.[0]?.code === MAINTENANCE_CODE;
+}
+
+export function maintenanceMessage(data) {
+  return data?.errors?.[0]?.message ?? DEFAULT_MAINTENANCE_MESSAGE;
+}
+
+/** خطای redirect نکست با throw کار می‌کند و نباید در catchهای عمومی بلعیده شود. */
+export function isNextRedirectError(error) {
+  return typeof error?.digest === "string" && error.digest.startsWith("NEXT_REDIRECT");
+}