perf(images): convert all images to WebP format

- Convert 248 PNG/JPG images to WebP (74% size reduction)
- Add convert-images-to-webp.sh script for future conversions
- Update all markdown image references to use .webp
- Update Astro component imports for default_img.webp

Before: 75.42 MB
After: 20.35 MB
Saved: 55.07 MB

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: ppippi-dev