Add SSDP/UPnP scanner to frontend status displays

Surface the SSDP/UPnP scanner alongside ARP and mDNS: a dedicated card on
the Status screen and a row in the consolidated Status card on Home. Mirrors
the existing mDNS implementation and reuses the generic ScannerStatusCard and
PolledValue infrastructure. Also maps the 'Ssdp' device-event scanner to the
"SSDP/UPnP" label.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/lib/model/device_event.dart

@@ -32,6 +32,7 @@ class DeviceEvent {
   String get scannerLabel => switch (scanner) {
     'Arp' => 'ARP',
     'Mdns' => 'mDNS',
+    'Ssdp' => 'SSDP/UPnP',
     _ => scanner,
   };
 }