Add production build config (standalone output, .env.production)

Pulled in from the parallel auth/cart work on origin/main that this
branch is superseding — these two pieces are independent of the auth
architecture and worth keeping: Next.js standalone output for a leaner
production deploy, and the production env file (public URLs only, no
secrets) it and the app read from.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

next.config.ts

@@ -1,6 +1,7 @@
 import type { NextConfig } from "next";
 
 const nextConfig: NextConfig = {
+  output: "standalone",
   allowedDevOrigins: ["sanihome.com", "localhost"],
 };