feat: implement user authentication flow with OTP-based signup, login, and verification endpoints

[Atanu2k4]

Summary

Removed the Resend email integration and replaced it with a standard SMTP configuration for handling application emails.

Related issue

Fixes #56

Changes

• Removed Resend email provider setup.
• Added SMTP configuration and implementation for sending emails.

Testing

• Verified that emails are successfully sent and delivered via the new SMTP provider.

Checklist

• Title follows the repository PR format
• The issue is linked with Fixes #56
• Testing notes are included

[github-actions]

✅ PR Validation Passed

Your PR title and description meet all requirements:

• Title follows conventional format
• All required sections are present
• Issue is properly linked (Remove resend and implement SMTP in the mail verification #56)