Replace merch placeholder images with real product photos

[pofallon]

Summary

The merch section uses stub placeholder images (12 bytes each) that need to be replaced with actual product photography or properly sized placeholder images.

Current State

apps/site-shell/public/images/merch/ contains stub files:

• logo-tee-hero.jpg (12 bytes)
• logo-tee-detail.jpg (12 bytes)
• dev-hoodie-hero.jpg (12 bytes)
• sticker-pack-hero.jpg (12 bytes)
• sticker-pack-spread.jpg (12 bytes)
• limited-cap-hero.jpg (12 bytes)

Required

1. Replace with actual product photography, OR
2. Use properly sized placeholder images (e.g., from placeholder.com or similar)
3. Recommended sizes:
   • Hero images: 800x600 (4:3 aspect ratio per ProductCard)
   • Gallery images: 800x800 (1:1 aspect ratio per ProductGallery)

Branch Context

Deferred from 011-merch-section implementation.