[codex] improve chapter navigation accessibility

[lsdragst-a11y]

Summary

• Marks active chapter navigation items with aria-current=location instead of the less specific true value.
• Adds aria-current to the active child chapter in the vertical rail.
• Adds regression tests for horizontal and vertical chapter navigation accessibility.
• Updates the frontend redesign plan to reflect completed stale component cleanup and the chapter aria-current work.

Validation

• npm test
• npx tsc --noEmit
• npm run lint
• npm run build