style: resolve landing lighthouse accessibility issues

[jb-thery]

Summary

• keep the Ragmir logo glitch effect with one real text node and pseudo-element decorative layers
• increase the hero demo tab index contrast to clear Lighthouse contrast checks

Validation

• pnpm lint
• pnpm --filter @jcode.labs/ragmir-landing check
• PUBLIC_RAGMIR_LANDING_URL=https://ragmir.com PUBLIC_RAGMIR_VERSION=2.4.0 pnpm --filter @jcode.labs/ragmir-landing build
• Lighthouse local mobile: Performance 96, Accessibility 100, SEO 100
• Lighthouse local desktop: Performance 100, Accessibility 100, SEO 100

Release impact

• landing-only style/accessibility change; no npm release expected