Email Subject Customization via Template Interface

It should be possible to customize the email subject directly from the template interface.

• By default, the template title can be used as the email subject.

• Template variables should be supported within the title to allow dynamic subject generation.

From the API perspective:

• If a template-id is provided, the subject field becomes optional.

• If both subject and template-id are provided, the explicitly defined subject in the API should override the template title.