Replace numeric explanation levels with emoji ladder

[becktothefuture]

Motivation

• Make the explanation ladder more visual and reduce cognitive effort by replacing numeric age labels with a five-emoji ladder while preserving the same progression and meaning.
• Keep the product name and overall UI/design unchanged, and ensure agents (Codex/Claude/Cursor) produce the emoji-labeled output by default.

Description

• Add a central EXPLANATION_LEVELS mapping and getLevelLabel in src/App.jsx, and update example data to use level keys instead of visible numeric age keys so the UI renders emoji labels.
• Update example rendering and typography-preview samples in src/App.jsx to show emoji labels (🧸, ✏️, 🎒, 📚, 🎓) and change wording that referenced ages to say “five visual levels” or similar.
• Replace the public downloadable skill at public/downloads/explain-it-like-im-5.md to require the emoji ladder (including the visible-output rule that answers must start with 🧸:) and update band guidance, guardrails, and final checks accordingly.
• Update supporting copy rules and guidance in COPY_RULES.md and public/downloads/references/learning-principles.md to use the emoji ladder; no other Codex/skill files were found in the repo beyond the public markdown skill.

Testing

• Ran the production build with npm run build, which completed successfully (Vite build finished and assets were generated).

---

Codex Task