Close the two remaining /gift funnel gaps

Add a hero image (rendered from the redesigned PDF's own cover) so the
opt-in page shows the actual resource instead of pure text, and make
the post-submit message PDF-specific with a working /gift-thanks
fallback link instead of a generic dead-end "check your inbox" line.

Both are opt-in per-page params (hero_image, success_message) so the
real drip-course page's rendering is unchanged.

layouts/partials/course-enroll-form.html

@@ -18,6 +18,7 @@
     var form = document.getElementById("enroll-form-{{ $id }}");
     var status = document.getElementById("enroll-status-{{ $id }}");
     var slug = {{ $page.Params.eec_slug }};
+    var successMessage = {{ ($page.Params.success_message | default "Check your inbox — let us know you're ready and we'll get started.") | markdownify }};
     form.addEventListener("submit", function (e) {
       e.preventDefault();
       if (form.website.value) return; // honeypot
@@ -32,7 +33,7 @@
         if (res.ok) {
           form.reset();
           form.hidden = true;
-          status.textContent = "Check your inbox — let us know you're ready and we'll get started.";
+          status.innerHTML = successMessage;
           return;
         }
         return res.text().then(function (msg) {