Adds credentials: include to the GraphQL fetch in preview.tsx when no
Bearer token is in the URL. Pairs with the soames-wordpress-theme
template_redirect hook that now sends the browser directly to
/preview/?id=<id>&type=post without a token, relying on the WordPress
session cookie for asPreview authentication.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>