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/widgets/scanners_status_card.dart

@@ -0,0 +1,167 @@
+import 'dart:async';
+
+import 'package:flutter/material.dart';
+import 'package:go_router/go_router.dart';
+
+import '../model/arp_scanner_status.dart';
+import '../model/mdns_scanner_status.dart';
+import '../utils/duration_formatter.dart';
+import '../utils/oott_api.dart';
+
+class ScannersStatusCard extends StatefulWidget {
+  const ScannersStatusCard({super.key});
+
+  @override
+  State<ScannersStatusCard> createState() => _ScannersStatusCardState();
+}
+
+class _ScannersStatusCardState extends State<ScannersStatusCard> {
+  ArpScannerStatus? _arpStatus;
+  String? _arpError;
+  MdnsScannerStatus? _mdnsStatus;
+  String? _mdnsError;
+  DateTime? _statusReceivedAt;
+  bool _isLoading = true;
+  Timer? _refreshTimer;
+  Timer? _tickTimer;
+
+  @override
+  void initState() {
+    super.initState();
+    _load();
+    _refreshTimer = Timer.periodic(const Duration(seconds: 5), (_) => _load());
+    _tickTimer = Timer.periodic(const Duration(seconds: 1), (_) {
+      if (mounted) setState(() {});
+    });
+  }
+
+  @override
+  void dispose() {
+    _refreshTimer?.cancel();
+    _tickTimer?.cancel();
+    super.dispose();
+  }
+
+  Future<void> _load() async {
+    ArpScannerStatus? arp;
+    String? arpError;
+    try {
+      arp = await BackendAPI.instance.getArpScannerStatus();
+    } catch (e) {
+      arpError = e.toString();
+    }
+
+    MdnsScannerStatus? mdns;
+    String? mdnsError;
+    try {
+      mdns = await BackendAPI.instance.getMdnsScannerStatus();
+    } catch (e) {
+      mdnsError = e.toString();
+    }
+
+    if (!mounted) return;
+    setState(() {
+      _arpStatus = arp;
+      _arpError = arpError;
+      _mdnsStatus = mdns;
+      _mdnsError = mdnsError;
+      _statusReceivedAt = DateTime.now();
+      _isLoading = false;
+    });
+  }
+
+  double get _elapsed => _statusReceivedAt != null
+      ? DateTime.now().difference(_statusReceivedAt!).inSeconds.toDouble()
+      : 0.0;
+
+  @override
+  Widget build(BuildContext context) {
+    if (_isLoading) {
+      return const Card(
+        child: Padding(
+          padding: EdgeInsets.all(16),
+          child: Center(child: CircularProgressIndicator()),
+        ),
+      );
+    }
+
+    final (arpColor, arpText) = _resolveArp();
+    final (mdnsColor, mdnsText) = _resolveMdns();
+
+    return Card(
+      clipBehavior: Clip.antiAlias,
+      child: InkWell(
+        onTap: () => context.go('/status'),
+        child: Padding(
+          padding: const EdgeInsets.all(16),
+          child: Column(
+            crossAxisAlignment: CrossAxisAlignment.start,
+            children: [
+              Text('Status', style: Theme.of(context).textTheme.titleMedium),
+              const SizedBox(height: 12),
+              _scannerRow(context, arpColor, 'ARP', arpText),
+              const SizedBox(height: 8),
+              _scannerRow(context, mdnsColor, 'mDNS', mdnsText),
+            ],
+          ),
+        ),
+      ),
+    );
+  }
+
+  Widget _scannerRow(
+    BuildContext context,
+    Color color,
+    String name,
+    String statusText,
+  ) {
+    return Row(
+      children: [
+        Icon(Icons.circle, color: color, size: 12),
+        const SizedBox(width: 10),
+        Expanded(
+          child: Text(name, style: Theme.of(context).textTheme.bodyMedium),
+        ),
+        const SizedBox(width: 8),
+        Text(
+          statusText,
+          style: Theme.of(context).textTheme.bodySmall?.copyWith(
+            color: Theme.of(context).colorScheme.outline,
+          ),
+        ),
+      ],
+    );
+  }
+
+  (Color, String) _resolveArp() {
+    if (_arpError != null || _arpStatus == null) {
+      return (Colors.red, 'Error');
+    }
+    if (_arpStatus!.isRunning) {
+      final secs = (_arpStatus!.runningForSeconds ?? 0) + _elapsed;
+      return (Colors.green, 'Running for ${formatSeconds(secs)}');
+    }
+    if (_arpStatus!.nextRunInSeconds != null) {
+      final remaining = (_arpStatus!.nextRunInSeconds! - _elapsed).clamp(
+        0.0,
+        double.infinity,
+      );
+      return (Colors.amber, 'Next run in ${formatSeconds(remaining)}');
+    }
+    return (Colors.grey, 'Not yet started');
+  }
+
+  (Color, String) _resolveMdns() {
+    if (_mdnsError != null || _mdnsStatus == null) {
+      return (Colors.red, 'Error');
+    }
+    if (_mdnsStatus!.isListening) {
+      if (_mdnsStatus!.lastDeviceSeenSecondsAgo != null) {
+        final secs = _mdnsStatus!.lastDeviceSeenSecondsAgo! + _elapsed;
+        return (Colors.green, 'Last device seen ${formatSeconds(secs)} ago');
+      }
+      return (Colors.green, 'No devices seen yet');
+    }
+    return (Colors.grey, 'Not yet started');
+  }
+}