Next 16 removed the `next lint` command, so `pnpm lint` was broken. Replace
.eslintrc.json with an ESLint 9 flat config (eslint.config.mjs) that spreads
eslint-config-next's native core-web-vitals flat config, and point the lint
script at `eslint .`.
Keep Next 16's newer react-hooks (v6) compiler-adjacent rules advisory (warn)
rather than blocking, since they flag working-but-non-optimal patterns across
the existing codebase. `pnpm lint` now passes (0 errors, warnings only).
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
