Improve landing page accessibility and mobile navigation

[lazyGPT07]

Summary

• add a keyboard-visible skip link and semantic main landmark
• make the mobile navigation expose state, move and restore focus, close on Escape, and lock background scrolling
• add accessible names to icon-only links and useful alternatives for informative images
• respect the operating system reduced-motion preference
• keep desktop and mobile navigation destinations consistent
• remove an unused import and fix the corrupted footer credit

Why

The landing page relies heavily on motion and icon-only controls, while the mobile menu does not expose its open state or expected keyboard behavior. These changes make the existing experience easier to navigate with a keyboard and assistive technology without changing the visual design or adding dependencies.

Validation

• npm run build passed, including TypeScript checking and static page generation
• git diff --check passed

npm run lint currently opens the upstream interactive ESLint setup prompt because no ESLint configuration has been committed, so it cannot run non-interactively without adding unrelated configuration.

[vercel]

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