Update signup status copy for the new readiness-email flow

Day one no longer sends immediately on signup — a readiness email
goes out first, and day one only sends once that's clicked. Update
the post-submit status text so it matches what actually happens.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

layouts/courses/single.html

@@ -35,7 +35,7 @@
         if (res.ok) {
           form.reset();
           form.hidden = true;
-          status.textContent = "Check your inbox — day one is on its way.";
+          status.textContent = "Check your inbox — let us know you're ready and we'll get started.";
           return;
         }
         return res.text().then(function (msg) {