fix: tolerate empty NEXT_PUBLIC_APP_URL during production build

lib/billing/paypal.ts

@@ -2,7 +2,7 @@ import type { PlanKey } from "./plans";
 
 const SANDBOX_BASE = "https://api-m.sandbox.paypal.com";
 const base = () => process.env.PAYPAL_API_BASE ?? SANDBOX_BASE;
-const appUrl = () => process.env.NEXT_PUBLIC_APP_URL ?? "http://localhost:3000";
+const appUrl = () => process.env.NEXT_PUBLIC_APP_URL || "http://localhost:3000";
 
 // Loudly warn (but don't throw — never break deploys) if PayPal is pointed at the
 // sandbox while running in production. The sandbox default is fine for dev.

lib/billing/stripe.ts

@@ -18,7 +18,7 @@ export function isStripeConfigured(): boolean {
   return !!process.env.STRIPE_SECRET_KEY;
 }
 
-const appUrl = () => process.env.NEXT_PUBLIC_APP_URL ?? "http://localhost:3000";
+const appUrl = () => process.env.NEXT_PUBLIC_APP_URL || "http://localhost:3000";
 
 /** Find-or-create the Stripe customer for a user and persist the id. */
 export async function ensureStripeCustomer(user: {