Enhancement/not found quicklinks

[AlonsoFi]

Closes #57

Enhances the 404 not-found page so users can recover from broken or expired URLs instead of hitting a dead end.

• Added with links to /contracts, /milestones, and /reputation
• Copy aligned with docs/COPYWRITING_GUIDE.md: direct, second-person, no technical jargon
• aria-hidden="true" on decorative 404 element
• Visually hidden h2 for screen reader heading navigation
• focus-visible:ring-2 on all links (WCAG 2.1 AA — SC 2.4.7)
• Responsive layout: stacked on mobile, inline from sm breakpoint
• Expanded tests from 1 → 11, 100% coverage
• Created docs/components/NotFound.md

Verified:

• npm test — 11/11 passed, 100% coverage
• npm run lint — no warnings or errors
• npm run build — compiled successfully

[drips-wave]

@AlonsoFi Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

You can now already apply to more issues while waiting for a review of this PR. Keep up the great work! 🚀

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