feat: remove ENABLE_MKTG_SITE guards from navbar templates

Marketing navigation links (HOW_IT_WORKS, COURSES, SCHOOLS) are now
always rendered in the unauthenticated nav bars. marketing_link() falls
back to '#' when a URL is not configured in MKTG_URLS.

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