Clean up site navigation

[Bmowville]

Summary

• make the project site navigation point clearly to Finder, Viewer, Challenge Index, Learning Paths, Roadmap, and GitHub
• make the hero primary action open the Challenge Viewer
• add a start section and footer links so users can move between Home, Finder, Viewer, Repository, and Contribute
• add breadcrumb, Home, previous, and next challenge links to the viewer
• prevent long run commands from causing horizontal overflow

Validation

• node --check docs/challenge-finder.js
• node --check docs/challenge-viewer.js
• python scripts/validate_challenges.py
• git diff --check
• VS Code diagnostics clean
• local desktop/mobile browser checks for homepage and viewer navigation with no horizontal overflow