fix: use EMAIL_FROM as primary sender address instead of EMAIL_SMTP_USER

[Zielgestalt]

The fromEmail config currently defaults to EMAIL_SMTP_USER, which is the SMTP login credential - not necessarily a valid sender address.

With some email services (e.g. Brevo), the SMTP username is completely different from the actual sender address (e.g. no-reply@example.com).

This change adds EMAIL_FROM as the first option in the fallback chain:

EMAIL_FROM → EMAIL_SMTP_USER → default