Fix: Make teal logos transparent

Converted white backgrounds to transparent for PolicyEngine logo PNG files.
This fixes the issue where logos didn't preserve transparency when converted from SVG.

Files updated:
- public/logo512.png
- public/main_logo.png
- social_cards/main_logo.png

🤖 Generated with Claude Code

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

Author: MaxGhenis