perf: use responsive images for theme preview mockups

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>

lib/simpleshop_theme_web/components/page_templates/home.html.heex

@@ -28,7 +28,7 @@
     <.image_text_section
       title="Made with passion, printed with care"
       description="Every design starts with an idea. We work with quality print partners to bring those ideas to life on premium products – from gallery-quality art prints to everyday essentials."
-      image_url="/mockups/mountain-sunrise-print-3.jpg"
+      image_url="/mockups/mountain-sunrise-print-3-800.webp"
       link_text="Learn more about the studio →"
       link_page="about"
       mode={@mode}