fix(email): report a missing sender address from the SMTP connection test

TestConnection skipped the empty-from guard that Send enforces, so with
no sender and no username configured the test issued the null reverse-path
and could report success against a lenient relay while every real
notification send kept failing with the missing-sender error. Guard the
test path the same way and surface a dedicated translated message.

internal/web/service/email/email.go

@@ -117,6 +117,9 @@ func (s *EmailService) TestConnection() SMTPTestResult {
 	if from == "" {
 		from = username
 	}
+	if from == "" {
+		return SMTPTestResult{false, "send", "smtpFromNotConfigured"}
+	}
 	from, fromName = resolveFrom(from, fromName)
 
 	recipients := parseRecipients(toStr)