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/).