feat(15-01): add sendSignerCompletionEmail to signing-mailer

- Sends plain-text email with signed document download link to signer
- Follows established createTransporter() + sendMail() pattern
- Subject: 'Signed copy ready: {documentName}', 72h expiry in body text

teressa-copeland-homes/src/lib/signing/signing-mailer.tsx

@@ -65,3 +65,22 @@ export async function sendAgentNotificationEmail(opts: {
     text: `${opts.clientName} has signed "${opts.documentName}" on ${formattedTime}.`,
   });
 }
+
+export async function sendSignerCompletionEmail(opts: {
+  to: string;
+  documentName: string;
+  downloadUrl: string;
+}): Promise<void> {
+  const transporter = createTransporter();
+  await transporter.sendMail({
+    from: '"Teressa Copeland Homes" <teressa@tcopelandhomes.com>',
+    to: opts.to,
+    subject: `Signed copy ready: ${opts.documentName}`,
+    text: [
+      `All parties have signed "${opts.documentName}".`,
+      '',
+      'Download your signed copy using the link below (expires in 72 hours):',
+      opts.downloadUrl,
+    ].join('\n'),
+  });
+}