Add multipart login emails with text/plain alternatives

[Sogl]

Summary

This PR adds proper multipart email support to the Login plugin by sending both HTML and plain-text parts for login-related emails.

Why

Some mail clients and filters handle HTML-only emails poorly. Adding a text/plain alternative improves compatibility, accessibility, and reliability (especially for activation/reset flows where the raw URL is important).

What changed

1. Updated classes/Email.php:

• replaced default single-template setup with multipart body assembly;
• keeps HTML template as primary part;
• automatically adds a plain-text part if a matching emails/login/<template>.txt.twig exists;
• added a small templateExists() helper using Twig loader.

2. Added plain-text templates:

• templates/emails/login/activate.txt.twig
• templates/emails/login/reset-password.txt.twig
• templates/emails/login/welcome.txt.twig
• templates/emails/login/notification.txt.twig
• templates/emails/login/invite.txt.twig

Each template reuses existing translation keys/context and includes the direct action link as plain text.

Backward compatibility

• If no .txt.twig exists for a template, behavior remains HTML-only.
• Existing HTML templates and email subjects are unchanged.

Notes

This change is intentionally minimal and limited to login email composition/templates only.

[rhukster]

I'll take a look at this when i circle back to Grav plugins.. probably next week.