Add PreviewData module that provides mock data for theme preview:
- 12 diverse mock products with realistic details and placeholder images
- 3 cart items with variants and quantities
- 6 customer testimonials with ratings
- 5 product categories with counts
- Smart fallback: uses real data when available, mock otherwise
All data designed to showcase theme customization options.
Includes 19 comprehensive tests.
🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>
