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/calendar/route.ts

@@ -1,11 +1,10 @@
 import { 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() {
   let upstream: Response;
   try {
-    upstream = await fetch(`${API_URL}/calendar/ics`, {
+    upstream = await fetch(apiUrl('/calendar/ics'), {
       headers: { Accept: 'text/calendar' },
       cache: 'no-store',
     });