refactor: reorganize email sending logic and centralize mail functionality

- Updated the `index.ts` file to export the `send` function from the new `mail.ts` file, improving code organization.
- Introduced `mail.ts` to encapsulate the email sending logic, including template rendering and error handling, enhancing modularity and maintainability.
- Implemented a structured approach for sending emails with templates or direct content, streamlining the email functionality.

src/mail/index.ts

@@ -1,5 +1,5 @@
 // Export the send function for direct import
-export { send } from './utils/send';
+export { send } from './mail';
 
 // Export mail templates
 export { EmailTemplates } from './emails';