panel: make the status page's "Full queue" a button

It was a bare link trailing the queue-summary sentence, while the two
other card actions on the same page (Re-check DNS, Reload configuration)
are buttons. Pull it out of the paragraph and give it the filled button
style through a new a.btn class — the same base rule a.danger already
used, so an action that happens to be a navigation still looks like
every other action.

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

internal/web/templates/status_body.html

@@ -30,8 +30,8 @@
     {{if .QueueError}}
     <p class="error">{{.QueueError}}</p>
     {{else}}
-    <p>{{if .QueueSummary}}{{.QueueSummary}}{{else}}Mail queue is empty.{{end}}
-    <a href="/queue">Full queue</a></p>
+    <p>{{if .QueueSummary}}{{.QueueSummary}}{{else}}Mail queue is empty.{{end}}</p>
+    <a class="btn" href="/queue">Full queue</a>
     {{end}}
   </div>