panel-redesign: Full backup and Help say why the queue is left out

Follows the design commit before it: the paragraph of the Full backup box and the Backup topic of Help.

  go test -count=1 ./internal/web/...: ok
  Backup page at 1280: no overflow, the box holds the longer paragraph.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/web/view/help.go

@@ -190,8 +190,10 @@ var serverTopics = []HelpTopic{
 	{ID: HelpBackup, Title: "Backup and moving a domain", Body: []Text{
 		Rich("Full backup downloads one archive of the whole instance: ", Code("data/"), " (the database, DKIM keys and application credentials), ",
 			Code("docker-compose.yml"), ", ", Code(".env"), " and ", Code("certs/"),
-			". The Postfix queue is not in it: mail still waiting for delivery is not carried over. Nor, usually, is the private key in ", Code("certs/"),
+			". The Postfix queue is not in it. The backup is taken while the server is running, and Postfix keeps moving messages through the queue, "+
-			", which the panel cannot read; ", Code("selfpost-backup"), " run in the container takes the key. It holds secrets, so leave Encrypt with a password ticked: the file is sealed as ", Code(".spbk"),
+				"so a copy could hold a message twice or miss it; mail still waiting for delivery is therefore not carried over, and a restored server "+
+				"starts with an empty queue. To keep the queue, stop the container and archive the project directory instead. The private key in ", Code("certs/"),
+			" is usually missing too, because the panel is not allowed to read it; ", Code("selfpost-backup"), " run in the container takes the key. It holds secrets, so leave Encrypt with a password ticked: the file is sealed as ", Code(".spbk"),
 			", the password is not stored, and without it nothing can be recovered. To restore, unpack it into an empty project directory (decrypt a ",
 			Code(".spbk"), " first with ", Code("selfpost-backup -decrypt"),
 			") and start the same SelfPost version that made it; the panel refuses data from another version. The reverse-proxy configuration and ",

internal/web/view/templates/backup.html

@@ -13,7 +13,7 @@
   <div class="column is-6">
 {{template "box_open" .Full}}
 {{template "body_open"}}
-<p class="mb-3"><code>data/</code> (database, DKIM keys, credentials), <code>docker-compose.yml</code>, <code>.env</code> and <code>certs/</code> in one archive, without the Postfix queue and without a certificate file the panel may not read. Restore into an empty project directory and start the <strong>same SelfPost version</strong>. The reverse-proxy vhost is not included.</p>
+<p class="mb-3"><code>data/</code> (database, DKIM keys, credentials), <code>docker-compose.yml</code>, <code>.env</code> and <code>certs/</code> in one archive. The Postfix queue is left out: it cannot be copied reliably while the server is running. So is a certificate file the panel may not read. Restore into an empty project directory and start the <strong>same SelfPost version</strong>. The reverse-proxy vhost is not included.</p>
 <form class="sp-form" method="post" action="{{.FullAction}}">
   <div class="field"><label class="checkbox"><input type="checkbox" name="encrypt" value="1"{{if .Encrypt}} checked{{end}} data-encrypt-toggle> Encrypt with a password (<code>.spbk</code>)</label></div>
   <div data-encrypt-fields>