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/dhcp_scanner_status.dart

@@ -0,0 +1,23 @@
+class DhcpScannerStatus {
+  final bool isListening;
+  final double? listeningForSeconds;
+  final int devicesSeen;
+  final double? lastDeviceSeenSecondsAgo;
+
+  const DhcpScannerStatus({
+    required this.isListening,
+    this.listeningForSeconds,
+    required this.devicesSeen,
+    this.lastDeviceSeenSecondsAgo,
+  });
+
+  factory DhcpScannerStatus.fromJson(Map<String, dynamic> json) {
+    return DhcpScannerStatus(
+      isListening: json['is_listening'] as bool,
+      listeningForSeconds: (json['listening_for_seconds'] as num?)?.toDouble(),
+      devicesSeen: (json['devices_seen'] as num).toInt(),
+      lastDeviceSeenSecondsAgo: (json['last_device_seen_seconds_ago'] as num?)
+          ?.toDouble(),
+    );
+  }
+}