Fix: Make all teal logos transparent

Converted white backgrounds to transparent for all 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
- src/images/logos/policyengine/teal.png
- src/images/logos/policyengine/profile/teal_bg.png

🤖 Generated with Claude Code

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

Author: MaxGhenis