feat(panel): adaptive monitoring poll intervals (v1.x closure phase 1)

Replace fixed 5 s hx-trigger polling with data-poll markers and panel.js
scheduling: 5 s while active, 30 s when idle, none when tab is hidden.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>
Co-authored-by: Cursor <cursoragent@cursor.com>

internal/web/templates/mail_queue_body.html

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