- Each message is role="article"; collapsed messages (consecutive from one
sender) now carry an aria-label with sender + time — previously a screen
reader heard only the body with no attribution (the biggest a11y gap).
Pure messageAriaLabel() reuses the existing time utils (+3 tests).
- Editing a message announces "Editing message from <sender>" (ariaLabel
threaded MessageEditor → CustomEditor; the main composer is unaffected).
- System emoji get role="img" + aria-label from the shortcode; custom
emoticons always have an accessible name.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
