Thread admin replies to original email via In-Reply-To/References headers

When replying to an inbound email that has a messageId, the outgoing
Resend payload now includes In-Reply-To and References headers so the
reply threads correctly in Gmail and other email clients.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

server/routes/contact.js

@@ -494,7 +494,13 @@ export function register(app) {
             { name: 'message_type', value: submission.messageType ?? 'general' },
             { name: 'submission_id', value: submission.id },
           ],
-          headers: { 'X-Contact-Submission-Id': submission.id },
+          headers: {
+            'X-Contact-Submission-Id': submission.id,
+            ...(submission.messageId ? {
+              'In-Reply-To': submission.messageId,
+              'References': submission.messageId,
+            } : {}),
+          },
           text,
           html,
         },