Sync web app OG banner with branded lego

The voice console's index.html uses /social-preview-lego.jpg for og:image
and twitter:image, so keep the public/ copy aligned with the branded
docs/social-preview-lego.jpg that drives the README hero.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: jvogan