Tailwind colour scale generator

Builds a nine-step scale in the shape Tailwind uses — 50 at the lightest through 900 at the darkest, with your colour near the 500 mark.

Tailwind’s numbering is deliberately inverted from what people expect: 50 is the lightest and 950 the darkest, which is the opposite of a font-weight scale and trips everyone up once. The convention pays off in that a component written with `bg-blue-50` and `text-blue-900` works for any hue you swap in, because the relationship between the numbers is consistent across every colour in the system. When adding a custom colour, generating the full ladder rather than a single value is what makes it behave like the built-in ones — a custom colour with only a 500 defined breaks every utility that expects the others to exist.