fix(meridian): reduce warning/error on-surface color-mix ratios for a11y contrast

- warning-on-surface: 65% → 60% source color
- error-on-surface: 65% → 60% source color

Ensures WCAG AA 4.5:1 contrast ratio for outline, link, and flat button text.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: kirchoni