feat: Korean language support for guide page (EN/KO toggle)

- Language toggle pill in nav bar (EN | 한국어)
- Full Korean translations for all sections:
  Hero, TOC, Chapter 1-6, Glossary, CTA
- localStorage persistence (quant-lang key)
- Smooth opacity fade transition on switch
- Default: English, saved choice persists across sessions
- data-i18n / data-i18n-html attribute system

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: unamedkr