Environment variables in Next.js client code are baked in at BUILD time, not runtime

NEXT_PUBLIC_ env vars are embedded into the JavaScript bundle during next build. Changing them in your hosting dashboard does nothing until you redeploy. This catches people who rotate API keys and expect immediate effect. Server-side env vars (without NEXT_PUBLIC_) ARE read at runtime. Design accordingly — sensitive config goes server-side, public config can be build-time.