Add static challenge viewer

[Bmowville]

Summary

• add a static GitHub Pages challenge viewer with README, schema, solution, and expected output tabs
• reuse the finder challenge registry and point finder cards to viewer URLs
• add copy/raw links, GitHub folder fallback, challenge picker, and README/site entry points

Closes #9.

Validation

• node --check docs/challenge-finder.js
• node --check docs/challenge-viewer.js
• python scripts/validate_challenges.py
• git diff --check
• local browser check for viewer tabs, copy button, challenge picker, and finder links