feat(blog): add clickable images that link to full-size versions

[CalvinAllen]

Summary

• Click any image in a blog post to open it in a styled lightbox
• Serves original unoptimized images for full-resolution viewing
• Lightbox features:
  • Header with title and close hint
  • Blue-bordered container with subtle glow
  • Prominent close button
  • Footer with instructions
  • Backdrop blur effect
• Close via X button, clicking outside image, or pressing Escape
• Hover effects (scale + shadow + zoom cursor) indicate images are clickable

How it works

1. copy-originals.js runs before dev/build and copies source images to public/originals/
2. Client-side script wraps prose images with click handlers
3. On click, maps the Astro-optimized URL back to the original and displays in lightbox

Test plan

• Run npm run dev and visit a post with images
• Verify hover effect on images (scale, shadow, zoom cursor)
• Click image and verify lightbox opens with full-size original
• Test closing via X button, clicking outside, and Escape key
• Verify original image is larger/higher quality than embedded version