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/.well-known/nostr.json/route.ts

@@ -1,10 +1,9 @@
 import { NextRequest, NextResponse } from 'next/server';
-
-const API_URL = process.env.NEXT_PUBLIC_API_URL || 'http://localhost:4000/api';
+import { apiUrl } from '@/lib/api-base';
 
 export async function GET(req: NextRequest) {
   const name = req.nextUrl.searchParams.get('name');
-  const upstream = new URL(`${API_URL}/nip05`);
+  const upstream = new URL(apiUrl('/nip05'));
   if (name) upstream.searchParams.set('name', name);
 
   const res = await fetch(upstream.toString(), { cache: 'no-store' });