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/single.html

@@ -0,0 +1,52 @@
+{{ define "main" }}
+<h1>{{ .Title }}</h1>
+{{ with .Params.cover_image }}
+<img src="{{ . }}" alt="Cover of {{ $.Title }}" style="max-width:300px;">
+{{ end }}
+{{ with .Params.author }}
+<p><span class="author-name">{{ . }}</span></p>
+{{ end }}
+{{ .Content }}
+{{ with .Params.stripe_price_id }}
+<div id="buy-btn-wrap">
+  {{ with $.Params.price_display }}<p><strong>{{ . }}</strong></p>{{ end }}
+  <button type="button" id="buy-btn" class="btn">Buy the ebook</button>
+</div>
+<div id="checkout-container"></div>
+<p id="buy-status" role="status" style="margin-top:1rem;"></p>
+
+<script src="https://js.stripe.com/v3/"></script>
+<script>
+  (function () {
+    var stripe = Stripe("{{ $.Site.Params.stripe_publishable_key }}");
+    var priceId = "{{ . }}";
+    var buyBtn = document.getElementById("buy-btn");
+    var buyWrap = document.getElementById("buy-btn-wrap");
+    var status = document.getElementById("buy-status");
+
+    buyBtn.addEventListener("click", function () {
+      buyBtn.disabled = true;
+      status.textContent = "Starting checkout…";
+      fetch("{{ $.Site.Params.bookshop_base_url }}/books/checkout", {
+        method: "POST",
+        headers: { "Content-Type": "application/json" },
+        body: JSON.stringify({ price_id: priceId })
+      }).then(function (res) {
+        if (!res.ok) { throw new Error("checkout session request failed"); }
+        return res.json();
+      }).then(function (data) {
+        status.textContent = "";
+        buyWrap.hidden = true;
+        return stripe.initEmbeddedCheckout({ clientSecret: data.clientSecret });
+      }).then(function (checkout) {
+        checkout.mount("#checkout-container");
+      }).catch(function () {
+        buyBtn.disabled = false;
+        buyWrap.hidden = false;
+        status.textContent = "Something went wrong starting checkout. Try again, or email hello@twosquirrelspress.com.";
+      });
+    });
+  })();
+</script>
+{{ end }}
+{{ end }}