[codex] refresh frontend visual design

[RolinShmily]

Summary

• redesign the site around an editorial guitar-score archive visual system
• refresh the homepage, sheet library, detail pages, about page, navigation, footer, and empty states
• improve sheet cards, search filters, PDF preview actions, typography, responsive layouts, and touch targets
• add shared SVG interface icons and reduced-motion/focus support

Why

The previous presentation used a flatter hierarchy and repeated card treatment, which made key content and actions harder to scan. This refresh gives the score previews, downloads, and browsing flow a clearer visual order while preserving the static-export architecture.

Impact

• four equal featured cards in a responsive two-column layout
• clearer search and attachment filtering
• stronger download and video call-to-action contrast
• improved keyboard focus, skip navigation, reduced-motion handling, and WCAG AA color contrast
• no content-schema, routing, or deployment changes

Validation

• pnpm typecheck
• pnpm lint
• pnpm test — 35 tests passed