Update theme preview to use optimized responsive images with modern
format support (AVIF/WebP with JPEG fallback).
- Change mockup URLs from .jpg to base paths for srcset generation
- Add source_width to preview products for proper variant selection
- Add responsive_image component with <picture> element
- Update image_text_section to use optimized 800px WebP variant
This ensures the theme preview loads optimal image formats and sizes,
matching the production responsive image behavior.
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
Jamey Greenwood
2c3d8f5647