feat: add scroll reveal animations

[sorinc03]

Summary

• add a small IntersectionObserver hook for one-time section reveal state
• animate Features, Architecture, For Developers, and Chains entries with staggered opacity/translate reveals
• respect prefers-reduced-motion by disabling transitions and showing content immediately

Fixes #3

Validation

• pnpm exec prettier --check src/hooks/useInView.ts src/index.css src/components/Features.tsx src/components/Architecture.tsx src/components/ForDevelopers.tsx src/components/Chains.tsx
• pnpm build
• git diff --check
• compared gzip output against origin/main: baseline CSS 4.16 kB / JS 64.68 kB, branch CSS 4.28 kB / JS 64.90 kB, roughly +0.34 kB gzip total
• verified in headless Chrome via CDP: below-fold items start hidden before scroll, each target section reveals with expected stagger delays, and reduced-motion emulation reports no transitions

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