Add /talk booking page with cal.com inline embed

The gift PDF's closing CTA promises a conversation, so it needs a
landing page to point to. Kept separate from /gift-thanks since the
PDF can be read/forwarded independent of any signup on this site.

assets/css/main.css

@@ -312,6 +312,20 @@ input:focus, textarea:focus {
   margin-top: 3rem;
 }
 
+/* Cal.com inline embed (layouts/partials/cal-embed.html) */
+.cal-embed {
+  width: 100%;
+  height: 700px;
+  border-radius: var(--radius);
+  overflow: hidden;
+  margin: 2rem 0;
+}
+@media (max-width: 30rem) {
+  .cal-embed {
+    height: 600px;
+  }
+}
+
 /* Home page: hero photo floats left, first list wraps beside it, second
    section clears below once the photo's height runs out. */
 main img {