fix: translate remaining Korean text to English in en/index.html

- Replace all hard-coded Korean card descriptions with English translations
- Fix data-i18n fallback text (phase5_title, footer links, motto)
- Change default language from 'ko' to 'en'
- Translate UI strings (progress counter label, reset confirm dialog)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: M1zz