style: :art: improve contrast for destructive color in dark mode

[flixlix]

Related to https://github.com/shadcn-ui/ui/pull/282, but it was closed and not fixed

Before

Not enough contrast with background

After

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[brhx]

For my own projects, I've decided to go with the following tailwind colors for appropriate contrast on text-based destructive states (and error messages):

bg-white text-red-600 dark:bg-zinc-950 dark:text-red-500

WCAG 2.0 level AA requires a contrast ratio of at least 4.5:1 for normal text.

• Light mode bg-white text-red-600: 4.82:1
• Dark mode bg-zinc-950 text-red-500: 5.28:1

Any lighter on the red in light mode (or darker in dark mode) results in the following contrast ratios:

• Light mode bg-white text-red-500: 3.76:1
• Dark mode bg-zinc-950 text-red-600: 4.11:1

Dark mode is better than light mode, but both are still below the minimum required contrast ratio of 4.5:1.