feat(blog): add 28 cover image style variations

- Add 5 color schemes: Blue, Purple, Green, Orange, Slate
- Each color has multiple angle variations (standard, reversed, steep, gentle)
- Random style selection by default, or specify with --style=N
- Generate all variations with --all-styles flag
- Blue: 6 variations with orange/gold/amber accents
- Purple: 6 variations with orange accent
- Green: 6 variations with orange/gold/amber accents
- Orange: 4 variations (no steep to avoid brown) with blue accent
- Slate: 6 variations with orange accent

Author: CalvinAllen