ESLint 9 defaults to flat config and eslint-config-next was pinned at 15
while Next is on 16, so eslint only ran with ESLINT_USE_FLAT_CONFIG=false
and next lint is gone on Next 16. Replace .eslintrc.json with a native
flat eslint.config.mjs (config-next 16 ships flat configs, so no
FlatCompat shim is needed), bump eslint-config-next to 16.2.6, add
@eslint/js and typescript-eslint as explicit devDeps for the recommended
rule sets, and point the lint script at eslint directly.
This only makes eslint runnable on modern tooling; it does not wire it
into CI. The same rules carry over (next/core-web-vitals, eslint and
typescript-eslint recommended, prettier, unused-imports)
c7f1bcfd0d