fix(builder): emit ContactForm successMessage as a data attribute (D1b)

successMessage was collected in props (and FormStylePanel already had a
live "Success Message" input for it) but toHtml never emitted it anywhere.
The form-sender relay delivers success via a full-page 303 redirect to
thankYouUrl or a hosted thanks.php page -- there is no in-page JS today
that could reveal an inline success element. Emit it as
data-whp-success-message (escaped) on the <form>: forward-compatible for a
future AJAX submission mode, without implying a live mechanism that
doesn't exist yet.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

craft/src/components/forms/ContactForm.tsx

@@ -211,8 +211,15 @@ ContactForm.craft = {
 
   const { marker, actionAttr, honeypot } = relayFormWiring(props.recipientEmail, props.thankYouUrl, props.formAction);
 
+  // The form-sender relay delivers success via a full-page 303 redirect
+  // (to thankYouUrl or a hosted thanks.php page) -- there is no in-page JS
+  // that reveals an inline success element today. Emit successMessage as a
+  // forward-compatible data attribute so a future AJAX/JS submission mode
+  // can read it, without implying a live mechanism that doesn't exist yet.
+  const successAttr = props.successMessage ? ` data-whp-success-message="${escapeAttr(props.successMessage)}"` : '';
+
   return {
-    html: `${marker}<form action="${actionAttr}" method="POST"${formStyle ? ` style="${formStyle}"` : ''}>
+    html: `${marker}<form action="${actionAttr}" method="POST"${successAttr}${formStyle ? ` style="${formStyle}"` : ''}>
 ${honeypot ? `  ${honeypot}\n` : ''}${fieldsHtml ? `  ${fieldsHtml}\n` : ''}  <button type="submit"${btnStyle ? ` style="${btnStyle}"` : ''}>${escapeHtml(props.submitText || 'Send Message')}</button>
 </form>`,
   };