Migrate UnitTesting.pptx to Slidev with images, notes, and layouts

- Extract images from PPTX via PDF→HTML pipeline
- Add speaker notes from PPTX XML as HTML comments
- Use default-image layout with circular corner images
- Use image-content, quote-alt, comparison layouts where appropriate
- Preserve emojis from original slides
- Add MigrationSkill.md with auto-detection rules
- Add default-image, quote-image layouts to theme
- Add size/image-position frontmatter to default-image
- Add size variants to agenda layout

Author: Laoujin