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).