feat(theme): interactive selector with live terminal preview

[Ansub]

Summary

• add a new interactive theme selector via boo theme select (aliases: choose, pick)
• show a live terminal-style preview panel on the right while browsing themes
• update theme list presentation to dark/light groups with main color palette swatches
• remove the old preview command from help and docs
• update README with the new selector workflow

Validation

• bash -n bin/boo
• bin/boo __theme_selector_preview clay
• bin/boo __theme_selector_preview abyss
• bin/boo theme select