CSS linting

Lint web/src/theme.css for correctness with stylelint + stylelint-config-standard. Prettier already handles formatting; stylelint covers correctness conventions (modern color notation, deprecated property values, etc.).

npm run lint:css        # check for issues — exits non-zero if any are found
npm run lint:css:fix    # auto-fix what stylelint can fix, then review the diff

Configuration lives in web/.stylelintrc.json. Purely formatting rules (declaration-block-single-line-max-declarations, *-empty-line-before) are disabled because the stylesheet uses a deliberate compact single-line style that Prettier preserves.