fix: send subscribe form as urlencoded so Customer.io records form_submit

The whats-new subscribe form posts to Customer.io's submit_action endpoint,
which expects application/x-www-form-urlencoded (the encoding native HTML
form POSTs use). Commit 6cce858 replaced the native submit with a fetch()
that sent FormData (multipart/form-data); the endpoint accepts the request
and returns the 302, but does not record a form_submit event, so no one is
added to the subscriber segment. The opaqueredirect response made the UI
report success regardless.

Switch the body to URLSearchParams so the request matches the encoding the
endpoint actually processes. Add regression tests pinning the body type.

Author: claude

fern/custom.js

@@ -158,12 +158,15 @@ function initializeSubscribeForm() {
     submitBtn.textContent = 'Submitting...';
 
     var formAction = form.getAttribute('action');
-    var formData = new FormData();
-    formData.append('email', email);
+    // Customer.io's submit_action endpoint expects application/x-www-form-urlencoded
+    // (the encoding native HTML form POSTs use). FormData sends multipart/form-data,
+    // which the endpoint accepts but does not record as a form_submit event.
+    var body = new URLSearchParams();
+    body.append('email', email);
 
     fetch(formAction, {
       method: 'POST',
-      body: formData,
+      body: body,
       redirect: 'manual',
     })
       .then(function (response) {

fern/custom.spec.js

@@ -103,6 +103,10 @@ assert(customJsContent.indexOf("redirect: 'manual'") !== -1,
   'custom.js uses fetch with redirect:manual to handle 302');
 assert(customJsContent.indexOf('opaqueredirect') !== -1,
   'custom.js checks for opaqueredirect response type');
+assert(customJsContent.indexOf('URLSearchParams') !== -1,
+  'custom.js sends body as application/x-www-form-urlencoded (URLSearchParams)');
+assert(customJsContent.indexOf('new FormData()') === -1,
+  'custom.js does not use FormData (multipart) for the Customer.io submit');
 assert(customJsContent.indexOf('subscribe-form-message') !== -1,
   'custom.js updates the message div');
 assert(customJsContent.indexOf('Thanks for subscribing') !== -1,