Add Books nav link and bookshop scaffold

Wires up bookshop_base_url and a "Books" nav entry ahead of Cate's
first title — content/books gets a coming-soon placeholder, and the
book single/ARC-redeem templates (copied from michaelferrara-site)
are ready to use once there's a real book page.

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 }}