Make message view's Back link set the mailbox page

This allows to go back and forth between the mailbox view and the
message view.

References: https://todo.sr.ht/~sircmpwn/koushin/22

imap.go

@@ -213,7 +213,7 @@ func listMessages(conn *imapclient.Client, mboxName string, page int) ([]imapMes
 
 	mbox := conn.Mailbox()
 	to := int(mbox.Messages) - page*messagesPerPage
-	from := to - messagesPerPage
+	from := to - messagesPerPage + 1
 	if from <= 0 {
 		from = 1
 	}

public/message.html

@@ -3,7 +3,9 @@
 <h1>koushin</h1>
 
 <p>
-	<a href="/mailbox/{{.Mailbox.Name | pathescape}}">Back</a>
+	<a href="/mailbox/{{.Mailbox.Name | pathescape}}?page={{.MailboxPage}}">
+		Back
+	</a>
 </p>
 
 <h2>

server.go

@@ -203,10 +203,11 @@ func handleGetPart(ctx *context, raw bool) error {
 	}
 
 	return ctx.Render(http.StatusOK, "message.html", map[string]interface{}{
-		"Mailbox":  mbox,
-		"Message":  msg,
-		"Body":     body,
-		"PartPath": partPathString,
+		"Mailbox":     mbox,
+		"Message":     msg,
+		"Body":        body,
+		"PartPath":    partPathString,
+		"MailboxPage": (mbox.Messages - msg.SeqNum) / messagesPerPage,
 	})
 }