feat: email templates and mailer service

[HelderMendes]

Introduces a stateless email notification system within the Next.js app.

Email templates

• confirm-email — account verification
• reset-password — time-sensitive password recovery link
• entity-created — confirmation when a user creates a new entity
• dataset-edit — notification when a dataset is edited
• admin-upload — alert for admin on new dataset uploads
• profile-update — confirmation on profile changes
• layout.ts — shared wrapper ensuring consistent branding across all templates

Mailer service
mail.ts — SMTP transport configuration, sends emails on-the-fly without persisting records to the database