feat(seo): add PWA icons for Google Search Console favicon visibility

- Add 8 PWA icons in /icons/ directory (72x72 through 512x512)
- Fixes site.webmanifest references that were returning 404
- Enables proper favicon display in Google Search results

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: jddunn