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.

assets/css/main.css

@@ -251,6 +251,15 @@ input:focus, textarea:focus {
   color: var(--fg-muted);
   margin-top: 0;
 }
+.course-hero-image {
+  float: none;
+  display: block;
+  width: 100%;
+  max-width: 320px;
+  height: auto;
+  margin: 1.5rem auto;
+  border-radius: var(--radius);
+}
 .course-value-prop {
   font-size: 1.05rem;
   margin: 1.5rem 0;