Add mDNS scanner status to Status and Home screens

Add an mDNS Scanner status card to the Status screen mirroring the ARP
card, and replace the ARP card on the Home screen with a combined Status
card that summarizes both scanners in one line each. Extract the shared
duration formatting helper into utils/duration_formatter.dart.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/lib/model/mdns_scanner_status.dart

@@ -0,0 +1,23 @@
+class MdnsScannerStatus {
+  final bool isListening;
+  final double? listeningForSeconds;
+  final int devicesSeen;
+  final double? lastDeviceSeenSecondsAgo;
+
+  const MdnsScannerStatus({
+    required this.isListening,
+    this.listeningForSeconds,
+    required this.devicesSeen,
+    this.lastDeviceSeenSecondsAgo,
+  });
+
+  factory MdnsScannerStatus.fromJson(Map<String, dynamic> json) {
+    return MdnsScannerStatus(
+      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(),
+    );
+  }
+}