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/sendlog.html

@@ -1,5 +1,5 @@
 {{define "content"}}
-<h1>Send log</h1>
+<h1>Deliveries</h1>
 
 <div class="card">
   <h2>Filter</h2>