feat(email): accept custom message headers in sendRaw

Lets callers set transport-level headers such as List-Unsubscribe /
List-Unsubscribe-Post. Also documents the puter-email driver's automatic
unsubscribe / report-abuse footer and the new `suppressed` result field
in the puter.js email module.

src/backend/clients/email/EmailClient.ts

@@ -50,6 +50,8 @@ export interface SendMailOptions {
     text?: string;
     replyTo?: string;
     attachments?: EmailAttachment[];
+    /** Extra message headers (e.g. List-Unsubscribe), passed to the transport. */
+    headers?: Record<string, string>;
 }
 
 export type EmailValidator = (email: string) => Promise<boolean> | boolean;

src/puter-js/src/modules/Email.js

@@ -22,6 +22,11 @@ import * as utils from '../lib/utils.js';
  *   });
  *
  * Positional form: `await puter.email.send(to, subject, body)`.
+ *
+ * Every mail automatically gets an unsubscribe / report-abuse footer.
+ * Recipients who unsubscribe are dropped from future sends — they come
+ * back in the result's `suppressed` array — and a send whose `to` list
+ * is entirely unsubscribed is rejected.
  */
 class Email {
     /**