panel: match monitoring URLs to their nav labels

/sendlog -> /deliveries, /queue -> /mail-queue, /logtail -> /system-log,
along with the HTMX polling fragments under each.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

internal/web/templates/queue_body.html

@@ -1,5 +1,5 @@
 {{define "queue_body"}}
-<div id="queue-body" hx-get="/queue/body" hx-trigger="every 5s" hx-swap="outerHTML">
+<div id="queue-body" hx-get="/mail-queue/body" hx-trigger="every 5s" hx-swap="outerHTML">
   {{if .Error}}<p class="error">{{.Error}}</p>{{end}}
   <span class="code">{{if .Output}}{{.Output}}{{else}}Queue is empty.{{end}}</span>
 </div>