Move Neon Tan's landing page from bookshop into Hugo

bookshop already supports this: ALLOWED_ORIGIN + CORS on POST
/arc/{code} exist specifically so a page on the brand's own domain can
call it cross-origin. There was no reason for bookshop to also render
the marketing/landing page itself — that put book copy inside a
generic, brand-agnostic Go template shared across every brand instead
of under normal editorial control like every other page on this site.

/books/neon-tan/ is now a plain Hugo page (content/books/neon-tan.md,
no longer draft-only/metadata-only — it's rendered for real via the new
layouts/books/single.html) with a small JS form
(layouts/partials/arc-redeem-form.html, mirroring the existing
course-enroll-form.html pattern) that POSTs form-encoded straight to
shop.michaelferrara.org/arc/neon-tan. bookshop's GET /arc/{code} page
rendering is no longer used for this campaign; only the POST redemption
and the download/Kindle/read/checkout endpoints still are.

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 }}

layouts/partials/arc-redeem-form.html

@@ -0,0 +1,61 @@
+{{ $id := .id }}
+{{ $page := .page }}
+<form id="arc-form-{{ $id }}" class="enroll-form">
+  <label for="arc-email-{{ $id }}">Email</label>
+  <input type="email" id="arc-email-{{ $id }}" name="email" required autocomplete="email" placeholder="you@example.com">
+
+  <div class="field-honeypot" aria-hidden="true">
+    <label for="arc-website-{{ $id }}">Leave this field empty</label>
+    <input type="text" id="arc-website-{{ $id }}" name="website" tabindex="-1" autocomplete="off">
+  </div>
+
+  <p class="field-subscribe">
+    <label><input type="checkbox" id="arc-subscribe-{{ $id }}" name="subscribe" value="1"> Keep me posted on new releases and giveaways</label>
+  </p>
+
+  <button type="submit" class="btn">{{ $page.Params.cta_text | default "Send me the story" }}</button>
+</form>
+<p id="arc-status-{{ $id }}" role="status" class="enroll-status"></p>
+
+<script>
+  (function () {
+    var form = document.getElementById("arc-form-{{ $id }}");
+    var status = document.getElementById("arc-status-{{ $id }}");
+    var arcURL = {{ $page.Site.Params.bookshop_base_url }} + "/arc/" + {{ $page.Params.arc_code }};
+    form.addEventListener("submit", function (e) {
+      e.preventDefault();
+      if (form.website.value) return; // honeypot
+      var submitBtn = form.querySelector("button[type=submit]");
+      submitBtn.disabled = true;
+      status.textContent = "Sending…";
+      var body = new URLSearchParams();
+      body.set("email", form.email.value);
+      if (form.subscribe.checked) body.set("subscribe", "1");
+      fetch(arcURL, {
+        method: "POST",
+        headers: { "Content-Type": "application/x-www-form-urlencoded" },
+        body: body.toString()
+      }).then(function (res) {
+        if (res.ok) {
+          form.reset();
+          form.hidden = true;
+          status.textContent = "Check your email — it's on its way.";
+          return;
+        }
+        submitBtn.disabled = false;
+        if (res.status === 400) {
+          status.textContent = "That doesn't look like a valid email address.";
+        } else if (res.status === 429) {
+          status.textContent = "Too many attempts — try again in a minute.";
+        } else if (res.status === 410) {
+          status.textContent = "This giveaway has ended.";
+        } else {
+          status.textContent = "Something went wrong. Try again in a moment.";
+        }
+      }).catch(function () {
+        submitBtn.disabled = false;
+        status.textContent = "Something went wrong. Try again in a moment.";
+      });
+    });
+  })();
+</script>