feat(qsl): credit line at the foot of the default QSL e-mail

"Designed & sent by OpsLog — https://github.com/GregTroar/OpsLog/releases/latest"
now closes the default body, so a fresh install sends it without being asked.

Put in the TEMPLATE, not appended at send time. Two consequences, both of them
the point: an operator who does not want it deletes the line once and it is
gone, and one who has already written their own body never sees it appear —
a stored template is returned verbatim and the default is only the fallback.
Appending it at send would have made it unremovable, which is not a thing to do
to somebody's outgoing mail.

The link is the human release page. updateCheckURL sits next to it in update.go
and answers JSON, which is the easy mistake here — a correspondent who clicks
the credit gets a download page, and a test says so.

app_qsl_designer.go

@@ -45,9 +45,20 @@ const appQSLCardSentField = "APP_OPSLOG_QSL_SENT"
 // (please send one). Independent of ADIF qsl_rcvd, like the sent field.
 const appQSLCardRcvdField = "APP_OPSLOG_QSL_RCVD"
 
+// qslCredit closes the default QSL e-mail: who made the card, and where the
+// recipient can get the same program.
+//
+// A DEFAULT, not a signature. It lives in the body template like every other
+// line, so an operator who does not want it deletes it once and it is gone —
+// and one who has already written their own body never sees it appear, because
+// a stored template is returned verbatim and the default is only the fallback.
+// Appending it at send time instead would have made it unremovable, which is
+// not a thing to do to someone's outgoing mail.
+const qslCredit = "--\nDesigned & sent by OpsLog — " + releasesPageURL
+
 const (
 	defaultQSLEmailSubject = "eQSL — {CALL} de {MYCALL}"
-	defaultQSLEmailBody    = "Hi,\n\nThank you for our QSO! Please find attached your eQSL card.\n\n{DATE}  ·  {BAND}  ·  {MODE}\n\n73,\n{MYCALL}"
+	defaultQSLEmailBody    = "Hi,\n\nThank you for our QSO! Please find attached your eQSL card.\n\n{DATE}  ·  {BAND}  ·  {MODE}\n\n73,\n{MYCALL}\n\n" + qslCredit
 )
 
 // qslDir is the root of all designer artifacts: templates/<id>/ and outbox/.