Finish the Neon Tan reader-magnet page

Fill in the gift page's copy (title, description, subhead), wire the
cover image into the hero and og:image/Twitter card meta, and point the
download links at the real bucket path. Also fixes a bug where every
page's meta description fell back to the site-wide bio instead of its
own.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

layouts/courses/single.html

@@ -1,6 +1,7 @@
 {{ define "main" }}
 <section class="course-hero">
   <h1>{{ .Title }}</h1>
+  {{ with .Params.cover_image }}<img class="course-cover" src="{{ . }}" alt="{{ $.Title }} cover">{{ end }}
   {{ with .Params.hero_subhead }}<p class="course-subhead">{{ . }}</p>{{ end }}
 
   {{ partial "course-enroll-form.html" (dict "id" "top" "page" .) }}