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/widgets/dhcp_scanner_card.dart

@@ -0,0 +1,47 @@
+import 'package:flutter/material.dart';
+
+import '../model/dhcp_scanner_status.dart';
+import '../theme/app_colors.dart';
+import '../utils/duration_formatter.dart';
+import '../utils/oott_api.dart';
+import 'scanner_status_card.dart';
+
+class DhcpScannerCard extends StatelessWidget {
+  const DhcpScannerCard({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    return ScannerStatusCard<DhcpScannerStatus>(
+      title: 'DHCP Scanner',
+      fetch: ({cancelToken}) =>
+          BackendAPI.instance.getDhcpScannerStatus(cancelToken: cancelToken),
+      resolver: _resolve,
+    );
+  }
+
+  static ScannerStatus _resolve(
+    BuildContext context,
+    DhcpScannerStatus status,
+    double elapsed,
+  ) {
+    if (!status.isListening) {
+      return (
+        color: Theme.of(context).colorScheme.outline,
+        label: 'Not yet started',
+        sublabels: [],
+      );
+    }
+    final sublabels = <String>[
+      status.listeningForSeconds != null
+          ? 'Listening for ${formatSeconds(status.listeningForSeconds! + elapsed)} · ${status.devicesSeen} devices seen'
+          : '${status.devicesSeen} devices seen',
+      if (status.lastDeviceSeenSecondsAgo != null)
+        'Last device ${formatSeconds(status.lastDeviceSeenSecondsAgo! + elapsed)} ago',
+    ];
+    return (
+      color: Theme.of(context).extension<AppColorExtension>()!.success,
+      label: 'Listening',
+      sublabels: sublabels,
+    );
+  }
+}