Add the new Grinchmas page (previous commit only captured deletions)

The prior commit's git add errored on an already-removed pathspec and
silently skipped every other file — this is the rest of that change:
content/books/grinchmas.md (real page, no longer draft-only),
layouts/books/single.html, layouts/partials/arc-redeem-form.html,
hugo.toml's bookshop_base_url-driven form target, and the homepage/nav
links pointing at /books/grinchmas/.

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

content/_index.md

@@ -5,5 +5,5 @@ description: "TODO: one-line description of Thistle & Moth"
 
 TODO: Thistle & Moth homepage copy goes here.
 
-Get a free story: [download the reader magnet](/gift). Writers: see our
+Get a free story: [download the reader magnet](/books/grinchmas/). Writers: see our
 [submission guidelines](/submissions).

content/books/grinchmas.md

@@ -3,5 +3,16 @@ title: "A Hallmark Grinchmas"
 author: "Marisol T. S. Kuhn"
 cover_image: "https://downloads.thistleandmoth.org/grinchmas/grinchmas-cover.png"
 description: "California native Bart Marley unexpectedly finds himself in the, to its mind perfect, midwestern town of Hallmark. One child refuses to give up on the stranger but is her innocence the Christmas miracle Bart needs?"
-draft: true
+
+# Label on the signup form's submit button.
+cta_text: "Send it to me"
+
+# Code of the bookshop ARC campaign this form redeems (see
+# reground-tools/arc-campaign.sh) — thistleandmoth.org/shop/arc/<arc_code>.
+arc_code: "grinchmas"
 ---
+California native Bart Marley unexpectedly finds himself in the, to its
+mind perfect, midwestern town of Hallmark. One child refuses to give up
+on the stranger but is her innocence the Christmas miracle Bart needs?
+
+Get *A Hallmark Grinchmas* free — enter your email below.