Add eslint and eslint-config-next as dev dependencies

The frontend had an .eslintrc.json config extending next/core-web-vitals
and next/typescript, but the actual eslint packages were never listed
in package.json. This meant `npm run lint` always failed.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: ambv