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>

services/api.js

@@ -2,6 +2,11 @@ import axios from "axios";
 import Cookies from "js-cookie";
 import { toast } from "react-toastify";
 import { getAccessToken, setAccessToken, clearAccessToken } from "@/lib/tokenStore";
+import {
+  isMaintenanceError,
+  maintenanceMessage,
+  MAINTENANCE_PATH,
+} from "@/lib/maintenance";
 
 function extractErrorMessage(error) {
   const errors = error?.response?.data?.errors;
@@ -74,6 +79,23 @@ api.interceptors.response.use(
   async (error) => {
     const original = error?.config;
 
+    // پیش از بررسی ۴۰۱: در حالت تعمیرات نباید مسیر refresh token طی شود و کاربر
+    // نباید logout شود. toast هم نمایش داده نمی‌شود چون صفحه‌ی تعمیرات جایگزین است.
+    if (isMaintenanceError(error)) {
+      if (typeof window !== "undefined") {
+        try {
+          sessionStorage.setItem(
+            "maintenance_message",
+            maintenanceMessage(error.response.data)
+          );
+        } catch { }
+        if (window.location.pathname !== MAINTENANCE_PATH) {
+          window.location.replace(MAINTENANCE_PATH);
+        }
+      }
+      return Promise.reject(error);
+    }
+
     if (
       error?.response?.status === 401 &&
       original?.requireAuth &&