feat: honor increased contrast preference

[Navesz]

Summary

• respond only to the explicit prefers-contrast: more setting
• strengthen secondary text, dividers, focus outlines and progress bars
• preserve unavailable states without relying on color alone
• document and test the contrast preference contract

Validation

• npm run lint
• npx tsc --noEmit
• npm test (140 tests)
• npm audit --omit=dev (0 vulnerabilities)