[ 21dev-org/21dev-org ] fix: change all text links from bitcoin-500 to bitcoin-700 for contrast
Global replacement of text-bitcoin-500 hover:underline with
text-bitcoin-700 hover:underline across all .astro files.
This fixes the remaining color contrast issues on /learn/, /tech/,
and /bips/ pages to meet WCAG AA 4.5:1 contrast requirement.