style(email): redesign email templates with dark fantasy theme (FRO-2)

- Replace basic light theme with immersive dark fantasy design
- Add gradient backgrounds and gold accent colors (#D4AF37)
- Include thematic emojis (castle, swords, shield icons)
- Add decorative borders and shadow effects on CTA buttons
- Translate all email content to French
- Use table-based layout for maximum email client compatibility
- Update unit tests to match French content

Author: pierrick-fonquerne