Use a branded landscape social card instead of the raw portrait cover

The book cover is a 1664x2432 portrait image, which crops badly into
og:image's expected 1200x630. Add a social_image field (a composited
card: cover centered on the site's parchment background) that og:image/
twitter:card prefer, falling back to cover_image if unset.

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

content/gift/index.md

@@ -11,9 +11,14 @@ hero_subhead: "This story, occasional emails when something new comes out and ot
 cta_text: "Send me the story"
 
 # Cover image, hosted in the public downloads bucket alongside the epub.
-# Used as the on-page hero image and for the og:image/Twitter card meta tags.
+# Used as the on-page hero image.
 cover_image: "https://downloads.michaelferrara.org/neon-tan/neon_tan_cover.jpg"
 
+# Optional: a landscape (1200x630) variant of the cover for social-share
+# previews (og:image/Twitter card). Falls back to cover_image if unset --
+# see layouts/_default/baseof.html.
+social_image: "https://downloads.michaelferrara.org/neon-tan/neon_tan_cover_social.jpg"
+
 # Direct link to the epub in the public downloads bucket — shown as an
 # on-page button immediately after a successful signup, independent of the
 # welcome email (see layouts/partials/course-enroll-form.html).