Add DHCP scanner to frontend status displays

Add a DHCP scanner card to the Status screen and a DHCP row to the
consolidated Status card on the Home screen, mirroring the existing
mDNS/SSDP implementations. Also label DHCP-discovered device events as
"DHCP" instead of the raw "Dhcp".

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

frontend/lib/model/device_event.dart

@@ -33,6 +33,7 @@ class DeviceEvent {
     'Arp' => 'ARP',
     'Mdns' => 'mDNS',
     'Ssdp' => 'SSDP/UPnP',
+    'Dhcp' => 'DHCP',
     _ => scanner,
   };
 }