fix: never re-enable SSID pool broadcast (crash guard) + top-bar health chip

Mode 'active' and the advertise toggle could re-enable the SSID-pool
broadcast that segfaults pineapd. active preset now skips ssidpool/enable
(advertise stays false), the advertise endpoint refuses with an
explanation when the pool is disabled, get_ap reports the real pool
state, and the PineAP overview disables the toggle with a notice. Added
a top-bar health chip (PINEAP OK / POOL OFF / PINEAPD DOWN) polled every
15s.

payload/user/remote_access/pager-webui/www/index.html

@@ -26,6 +26,7 @@
     <span id="brand-text" class="brand-text">Mark VIII</span>
     <span class="toolbar-spacer"></span>
     <span id="live-status"></span>
+    <span id="health-status" class="health-chip"></span>
     <div class="toolbar-action">
       <button id="notifications-btn" class="toolbar-icon-btn" type="button" title="Notifications"
               aria-label="Notifications" aria-haspopup="menu" aria-controls="notifications-menu" aria-expanded="false"></button>