Fix: Make all teal logos transparent (#2799)

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