Replace plain-text message rendering with a rich MessageBody component
that supports bold, italic, inline code, fenced code blocks, lists,
headers, auto-linked URLs, @mention pills (linking to /dm/{name}), and
#channel pills (linking to /channels/{name}). Input is HTML-sanitized
before processing to prevent XSS. Updated all 5 rendering locations:
channels, DMs, conversations, MessageList, and ThreadPanel.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
