web/lint: flag non-canonical tailwind classes with eslint
the tailwind language server's suggestCanonicalClasses diagnostic only
runs inside an editor, so nothing catches these in CI.
eslint-plugin-better-tailwindcss exposes the same canonicalization as a
lint rule.
rootFontSize matters: without it the rule silently skips every px-based
rewrite, which is most of them.
Signed-off-by: eti <eti@eti.tf>