chore: Add PNG versions of avatars for GitHub social previews

- Convert all 11 SVG avatars to PNG (1280x640)
- Fix rhodium-standard.svg and zotero-generic.svg for conversion
- PNGs ready for GitHub social preview upload

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: Jonathan D.A. Jewell