fix: darken --text-muted to meet WCAG AA contrast on light backgrounds

[MaximeGaudin]

Summary

• --text-muted (#6b7f8e) is used for small body text (10–13.5px: .rule-summary, .badge.plain, .back-link, .citation span, table th) on the light paper backgrounds. Measured via the WCAG relative-luminance formula, its worst case is 3.51:1 (on the body gradient's darkest stop #e4edf3) and 3.75:1 on --paper — below the 4.5:1 AA minimum for normal text.
• Darkened --text-muted to #576874, which clears 4.5:1 on every light background it sits on: 4.87:1 on #e4edf3, 5.20:1 on --paper, 5.51:1 on --paper-raised.

Not changed (verified already passing)

• The issue also flagged #7f93a1 at .console-bar (line 522) and footer p (line 902) as 2.87:1. That computation compared them against --paper, but both actually sit on dark panes: .console background #060c11 and footer background #071016. On their real backgrounds the contrast is 6.17:1 and 6.02:1 — comfortably above AA. No change needed.
• --signal-bright (#14c4b5) is left as-is: the issue explicitly does not assert it as a failure (its traced usages sit on dark console/hero panels) and flags it only as a devtools follow-up.

Fixes #120

Test plan

• Computed WCAG contrast ratios for the old and new --text-muted against every light background in the palette (#e4edf3, #eef4f7, #f7fafc); the new value is >= 4.5:1 in all cases, and the #7f93a1 / --text-secondary pairs were re-verified as passing.
• pnpm run build in apps/docs: 48 pages built.
• pnpm run lint (biome + astro check + check-no-todos): 0 errors, 0 warnings, clean.
• CI green on the PR