Finalize Rivets design and challenge learning experience

[triggeredcode]

Summary

This PR publishes the 47 local commits accumulated since the current main branch. It brings the Rivets visual direction and challenge learning experience together for review.

• Introduces the modular Rivets mark, updated typography, exhibition palette, and cohesive home, challenge library, dashboard, and leaderboard layouts.
• Adds the autonomous storage cutaway and crash recovery illustration. Removes the flashing green checkmarks and earlier unused homepage demos.
• Gives challenge families distinct artwork and balances storage card colors across each two-column row.
• Includes the earlier mobile workspace, learning content, execution feedback, and open-source copy improvements already committed on this branch.
• Updates the active design guide to match the finished template.
• Pins patched sharp and source-map-js transitive releases after the initial GitHub web CI security audit identified two production advisories.

Verification

• Repository hygiene checks passed.
• Production dependency audit passed with zero vulnerabilities after a clean install.
• TypeScript, ESLint, and production build passed.
• 95 unit tests passed.
• 75 browser tests passed against the local production preview, including real local arena submissions, reduced-motion behavior, and five viewport sizes across eight page types.

The full audit still reports a development-only braces advisory through the ESLint toolchain. The GitHub advisory currently lists no patched release; npm proposes a breaking ESLint configuration downgrade. Production dependencies audit cleanly.

Review focus

Check the hero motion and recovery illustration on the landing page, the color rhythm in the storage challenge rows, and the challenge workspace on phone and desktop.