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>

content/gift-thanks/index.md

@@ -1,8 +1,12 @@
 ---
-title: "Thanks — here's your story"
-description: "Download your free short story."
+title: "Thanks! Here's your story."
+description: "Enjoy Neon Tan!"
 ---
 
-Thanks for signing up — here's your copy.
+Here's your copy of *Neon Tan*.
 
-[Download the story](https://downloads.michaelferrara.org/TODO-story-filename.epub)
+[Download the story](https://downloads.michaelferrara.org/neon-tan/neon_tan.epub)
+
+Talk soon,
+
+Michael