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.

update.go

@@ -23,6 +23,11 @@ import (
 // build (the exe lives there; source stays on Gitea). Adjust the repo if needed.
 const updateCheckURL = "https://api.github.com/repos/GregTroar/OpsLog/releases/latest"
 
+// releasesPageURL is the same release, for people rather than for the updater:
+// the API address above answers JSON, so it is not something to put in front of
+// an operator who followed a link out of a QSL e-mail.
+const releasesPageURL = "https://github.com/GregTroar/OpsLog/releases/latest"
+
 // UpdateInfo is the result of the version check.
 type UpdateInfo struct {
 	Current     string `json:"current"`      // this build's version (appVersion)
@@ -367,8 +372,8 @@ func cleanupOldUpdateBinary() {
 	if err != nil {
 		return
 	}
-	_ = os.Remove(exe + ".old")  // the old fixed name
-	_ = os.Remove(exe + ".new")  // a deferred swap that has been applied
+	_ = os.Remove(exe + ".old") // the old fixed name
+	_ = os.Remove(exe + ".new") // a deferred swap that has been applied
 	matches, err := filepath.Glob(exe + ".old-*")
 	if err != nil {
 		return