Add Stripe embedded checkout to book pages

Replaces the gumroad_url stub with an on-site embedded checkout (buy
button -> POST to the bookshop backend -> mounted Stripe iframe),
following the existing contact/submissions form's fetch/status-message
pattern. Adds thank-you and resend-link pages, an example book template
(draft), and updates the privacy policy and terms to honestly disclose
Stripe as a third-party payment processor.

layouts/books/book-resend.html

@@ -0,0 +1,43 @@
+{{ define "main" }}
+<h1>{{ .Title }}</h1>
+{{ .Content }}
+
+<form id="resend-form" style="margin-top:2rem;">
+  <label for="email">Email</label>
+  <input type="email" id="email" name="email" required autocomplete="email">
+
+  <div class="field-honeypot" aria-hidden="true">
+    <label for="website">Leave this field empty</label>
+    <input type="text" id="website" name="website" tabindex="-1" autocomplete="off">
+  </div>
+
+  <button type="submit" class="btn">Resend my link</button>
+</form>
+<p id="resend-status" role="status" style="margin-top:1rem;"></p>
+
+<script>
+  (function () {
+    var form = document.getElementById("resend-form");
+    var status = document.getElementById("resend-status");
+    form.addEventListener("submit", function (e) {
+      e.preventDefault();
+      var payload = { email: form.email.value, website: form.website.value };
+      var submitBtn = form.querySelector("button[type=submit]");
+      submitBtn.disabled = true;
+      status.textContent = "Sending…";
+      fetch("{{ .Site.Params.bookshop_base_url }}/books/resend", {
+        method: "POST",
+        headers: { "Content-Type": "application/json" },
+        body: JSON.stringify(payload)
+      }).then(function () {
+        form.reset();
+        status.textContent = "If that email has a purchase on file, a new link is on its way.";
+      }).catch(function () {
+        status.textContent = "Something went wrong. Try again, or email hello@twosquirrelspress.com directly.";
+      }).finally(function () {
+        submitBtn.disabled = false;
+      });
+    });
+  })();
+</script>
+{{ end }}