panel: clearer nav labels and card headings, danger-style Sign out

Rename technical labels (postqueue -p, mail.log tail, Send log/Queue/Log
in nav) to plain descriptions, and style Sign out as a deliberate danger
action consistent with other destructive buttons.

internal/web/templates/queue.html

@@ -2,7 +2,7 @@
 <h1>Mail queue</h1>
 
 <div class="card">
-  <h2>postqueue -p</h2>
+  <h2>Pending messages</h2>
   {{template "queue_body" .}}
 </div>
 {{end}}