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>

assets/css/main.css

@@ -385,6 +385,15 @@ textarea {
   text-align: center;
 }
 
+.course-cover {
+  max-width: 14rem;
+  width: 100%;
+  height: auto;
+  border-radius: 4px;
+  box-shadow: 0 4px 16px rgba(0, 0, 0, 0.2);
+  margin: 0 auto var(--space-3);
+}
+
 .course-subhead {
   font-style: italic;
   color: var(--color-brass);