feat: add alternatives comparison section

[sorinc03]

Summary

• Add a sourced comparison section explaining where Wraith fits against mixer-style flows, Aztec/private rollups, RAILGUN, and transparent payments.
• Include an accessible responsive comparison table plus prose cards with official source links.
• Link the comparison from the Features section and footer, with hash scrolling that respects the fixed header.

Validation

• pnpm exec prettier --check src/App.tsx src/components/Compare.tsx src/components/Features.tsx src/components/Footer.tsx
• pnpm build
• git diff --check
• Headless Chrome render check at /#compare for desktop 1440px and mobile 390px; mobile table scrolls inside its container with document scrollWidth matching viewport.

Fixes #11

[vercel]

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