Move Neon Tan's landing page from bookshop into Hugo

bookshop already supports this: ALLOWED_ORIGIN + CORS on POST
/arc/{code} exist specifically so a page on the brand's own domain can
call it cross-origin. There was no reason for bookshop to also render
the marketing/landing page itself — that put book copy inside a
generic, brand-agnostic Go template shared across every brand instead
of under normal editorial control like every other page on this site.

/books/neon-tan/ is now a plain Hugo page (content/books/neon-tan.md,
no longer draft-only/metadata-only — it's rendered for real via the new
layouts/books/single.html) with a small JS form
(layouts/partials/arc-redeem-form.html, mirroring the existing
course-enroll-form.html pattern) that POSTs form-encoded straight to
shop.michaelferrara.org/arc/neon-tan. bookshop's GET /arc/{code} page
rendering is no longer used for this campaign; only the POST redemption
and the download/Kindle/read/checkout endpoints still are.

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

content/_index.md

@@ -5,4 +5,4 @@ description: "TODO: one-line author bio/description"
 
 TODO: Michael's homepage copy goes here.
 
-Get a free short story: [download the reader magnet](https://shop.michaelferrara.org/arc/neon-tan).
+Get a free short story: [download the reader magnet](/books/neon-tan/).

content/books/neon-tan.md

@@ -2,6 +2,20 @@
 title: "Neon Tan"
 author: "Michael Ferrara"
 cover_image: "https://downloads.michaelferrara.org/neon-tan/neon_tan_cover.jpg"
-description: "The world is stranger than it looks and more meaningful than it lets on. You know this. Sign up to my mailing list and I'll send you a story about an AI, a wisecracking seagull, and a song that needs to be heard."
-draft: true
+description: "An AI's trademark silver-blue suit isn't exactly beach-town vibes, but he's determined to land his song a spot on the weekly top 40 countdown anyway. After a string of failed attempts to get noticed by the countdown's enigmatic curator DJ, he and his wisecracking seagull sidekick stumble into an open mic night just as a malware glitch hits, forcing him to trade analysis for improvisation, rallying tourists into one spontaneous singalong to discover connection was never something you could calculate."
+
+# Optional italic line under the title, for handling an objection up front.
+hero_subhead: "A free short story, plus occasional emails when something new comes out."
+
+# Label on the signup form's submit button.
+cta_text: "Send me the story"
+
+# Code of the bookshop ARC campaign this form redeems (see
+# reground-tools/arc-campaign.sh) — shop.michaelferrara.org/arc/<arc_code>.
+arc_code: "neon-tan"
 ---
+The world is stranger than it looks and more meaningful than it lets on.
+
+You know this.
+
+Sign up to my mailing list and I'll send you a story about an AI, a wisecracking seagull, and a song that needs to be heard.