fix(frontend): resolve API base URL at request time for production

Use same-origin /api in the browser so builds are not stuck with baked-in
localhost. Server-side fetches use INTERNAL_API_URL, NEXT_PUBLIC_API_URL, or
loopback. Centralize logic in lib/api-base.ts.

Made-with: Cursor

frontend/app/sitemap.ts

@@ -1,12 +1,12 @@
 import type { MetadataRoute } from "next";
+import { apiUrl } from "@/lib/api-base";
 
 const siteUrl =
   process.env.NEXT_PUBLIC_SITE_URL || "https://belgianbitcoinembassy.org";
-const apiUrl = process.env.NEXT_PUBLIC_API_URL || "http://localhost:4000/api";
 
 async function fetchJson<T>(path: string): Promise<T | null> {
   try {
-    const res = await fetch(`${apiUrl}${path}`, { next: { revalidate: 3600 } });
+    const res = await fetch(apiUrl(path), { next: { revalidate: 3600 } });
     if (!res.ok) return null;
     return res.json();
   } catch {