fix: Resend rejects noreply@localhost as from address

When NEXTAUTH_URL=http://localhost:3000, the hostname resolves to
'localhost' which Resend rejects. Fall back to onboarding@resend.dev
when hostname is localhost.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

memento-note/lib/mail.ts

@@ -61,8 +61,9 @@ async function sendViaResend(apiKey: string, { to, subject, html, attachments }:
     const { Resend } = await import('resend');
     const resend = new Resend(apiKey);
 
-    const from = process.env.NEXTAUTH_URL
-      ? `Memento <noreply@${new URL(process.env.NEXTAUTH_URL).hostname}>`
+    const hostname = process.env.NEXTAUTH_URL ? new URL(process.env.NEXTAUTH_URL).hostname : '';
+    const from = hostname && hostname !== 'localhost'
+      ? `Memento <noreply@${hostname}>`
       : 'Memento <onboarding@resend.dev>';
 
     // Resend supports attachments with inline content