Root cause: Vercel wasn't processing Tailwind through @tailwindcss/postcss,
serving raw CSS with zero utility classes (23KB theme-only vs 53KB with
utilities). The source() directive was output literally as @media source().

Fixes per official Tailwind v4 + Next.js setup:
- Switch postcss.config.js (CJS) → postcss.config.mjs (ESM)
- Remove autoprefixer (built into Tailwind v4)
- Use plain @import "tailwindcss" (auto source detection)
- Move tailwindcss + @tailwindcss/postcss to dependencies