style: Generate transparent differ-white.png directly from SVG

The previous `magick` fuzz approach might have left artifacts or failed to fully handle anti-aliasing. This converts the clean `differ-white.svg` directly to a high-quality `differ-white.png` with a true transparent (`none`) background.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: claude