Add previewImage assets to Pilot section schemas

[hta218]

Parent

https://github.com/Weaverse/builder/issues/2682

Context

Pilot should be the first theme to adopt static section preview images. Designer-provided assets will be wired into section schemas via previewImage once the SDK and Studio support are ready.

Scope

• Add previewImage URLs to high-value Pilot section schemas.
• Use stable, cacheable CDN URLs with versioned filenames.
• Start with the most common sections:
  • Hero image / hero video
  • Image with text
  • Featured products
  • Featured collections
  • Collection list
  • Testimonials / reviews
  • Columns / gallery / countdown / contact sections
• Coordinate with the design task for final assets.

Acceptance criteria

• Pilot section schemas expose previewImage for the selected core sections.
• Studio Add Section picker displays images for those sections.
• Sections without assets continue to use iframe fallback.
• No storefront rendering changes outside schema metadata.