fix: remove white background from all logo PNGs (#389)

The Gemini-generated logos had white backgrounds that looked wrong
on the dark-themed website. Removed white/near-white pixels and
set them to transparent using Pillow.

Co-authored-by: Ajit Pratap Singh <ajitpratapsingh@Ajits-Mac-mini-2655.local>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: 3 people