I built a fitness SaaS in Next.js — here are the 3 problems that ate all my time

I'm a CS student, and for the last few months I've been building a fitness app in my spare time — the kind of thing where you tell yourself "it's just a workout logger" and three months later you've got offline sync, a food database, Stripe subscriptions and an AI coach. Classic scope creep, but I learned a lot. Here are the three things that actually ate my time, and how I ended up solving them. (Stack, if you care: Next.js 16 App Router, React 19, Prisma 7 + Postgres, Auth.js v5, Tailwind 4.)