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/blog/[slug]/page.tsx

@@ -1,12 +1,11 @@
 import type { Metadata } from "next";
 import BlogPostClient from "./BlogPostClient";
 import { BlogPostingJsonLd, BreadcrumbJsonLd } from "@/components/public/JsonLd";
-
-const apiUrl = process.env.NEXT_PUBLIC_API_URL || "http://localhost:4000/api";
+import { apiUrl } from "@/lib/api-base";
 
 async function fetchPost(slug: string) {
   try {
-    const res = await fetch(`${apiUrl}/posts/${slug}`, {
+    const res = await fetch(apiUrl(`/posts/${slug}`), {
       next: { revalidate: 300 },
     });
     if (!res.ok) return null;