fix(reply): account sender for lists

[FromSi]

What?

This PR changes the SMTP envelope sender used when sending emails.

Previously, Matcha used account.GetFetchEmail() for SMTP MAIL FROM. Now it uses account.Email instead.

In practice:

Before:
MAIL FROM:<fetch_email>

After:
MAIL FROM:<account.email>

The visible message From: header is unchanged and still uses the existing send-as logic.

Why?

Closes #1274

fetch_email is used by Matcha to decide which incoming messages belong to an account or account tab. It can be different from the authenticated email address, especially for aliases, catch-all mailboxes, forwarded mail, or mailing list workflows.

Using fetch_email as SMTP MAIL FROM is incorrect because SMTP servers usually validate the envelope sender against the authenticated account. If fetch_email points to a mailing list or another non-authorized address, servers such as Postfix can reject the message with errors like 554 Transaction failed.

Example broken case:

account.email = user@example.org
fetch_email   = alias@example.org

Old behavior:
MAIL FROM:<alias@example.org>

The SMTP server may reject this because user@gmail.com is not allowed to send mail on behalf of alias@example.org.

With this fix:

MAIL FROM:<user@example.org>

So replies to mailing list messages are sent using the authenticated account as the SMTP envelope sender, while the normal reply recipient and visible headers continue to behave as before.