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>

layouts/books/single.html

@@ -0,0 +1,11 @@
+{{ define "main" }}
+<section class="book-hero">
+  <h1>{{ .Title }}</h1>
+  {{ with .Params.cover_image }}<img class="book-cover" src="{{ . }}" alt="{{ $.Title }} cover">{{ end }}
+  {{ with .Params.hero_subhead }}<p class="book-subhead">{{ . }}</p>{{ end }}
+
+  {{ .Content }}
+
+  {{ partial "arc-redeem-form.html" (dict "id" "top" "page" .) }}
+</section>
+{{ end }}