Add styled HTML email templates for market submission notifications
and magic link authentication, matching the frontend's forest green
and gold design language.
- Template engine using go:embed with clone-per-content pattern
- Multipart/alternative MIME (text/plain + text/html) in SMTPSender
- Base layout: dark green header, gold accent, parchment tones
- Market submission template with details table and admin CTA
- Magic link template with CTA button and fallback URL
- Wire email sending into MagicLinkHandler (replaces TODO)
- Add FRONTEND_URL config for template links
vikingowl
e07f4c4c64