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>

hugo.toml

@@ -32,7 +32,7 @@ disableKinds = ["taxonomy", "term"]
 
 [[menu.main]]
   name = "Free Book"
-  url = "/free-book/"
+  url = "/books/grinchmas/"
   weight = 30
 
 [[menu.main]]