Harden post-launch metadata and accessibility

[advaitmb]

Summary

• use the existing favicon asset and add the missing homepage canonical URL
• correct top-level team and publication heading hierarchy without changing card styling
• fail exported builds on heading-level jumps or a missing favicon
• record the first React deployment and exact rollback state in the maintainer runbook

Verification

• npm run check
• mobile and desktop production smoke tests
• publication search empty state and reset
• keyboard focus starts on the skip link

Refs #26 and #29.