Refactor frontend: extract widgets and eliminate duplicated scanner state

- Convert ArpScannerCard to a self-managing StatefulWidget (owns its own 5s refresh + 1s tick timers), removing the duplicated state management that existed in both HomeScreen and StatusScreen
- Extract NotificationCard to home/notification_card.dart
- Extract DeviceSummaryCard (with its 1-min refresh timer) to widgets/device_summary_card.dart
- HomeScreen drops from 515 to 253 lines; StatusScreen from 82 to 19 lines

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

frontend/lib/status/status_screen.dart

@@ -1,67 +1,10 @@
-import 'dart:async';
-
 import 'package:flutter/material.dart';
-import 'package:frontend/model/arp_scanner_status.dart';
-import 'package:frontend/utils/oott_api.dart';
-import 'package:frontend/widgets/arp_scanner_card.dart';
 
-class StatusScreen extends StatefulWidget {
+import '../widgets/arp_scanner_card.dart';
+
+class StatusScreen extends StatelessWidget {
   const StatusScreen({super.key});
 
-  @override
-  State<StatusScreen> createState() => _StatusScreenState();
-}
-
-class _StatusScreenState extends State<StatusScreen> {
-  ArpScannerStatus? _status;
-  DateTime? _statusReceivedAt;
-  bool _isLoading = true;
-  String? _error;
-  Timer? _refreshTimer;
-  Timer? _tickTimer;
-
-  @override
-  void initState() {
-    super.initState();
-    _loadStatus();
-    _refreshTimer = Timer.periodic(
-      const Duration(seconds: 5),
-      (_) => _loadStatus(),
-    );
-    _tickTimer = Timer.periodic(
-      const Duration(seconds: 1),
-      (_) {
-        if (mounted) setState(() {});
-      },
-    );
-  }
-
-  @override
-  void dispose() {
-    _refreshTimer?.cancel();
-    _tickTimer?.cancel();
-    super.dispose();
-  }
-
-  Future<void> _loadStatus() async {
-    try {
-      final status = await BackendAPI.instance.getArpScannerStatus();
-      if (!mounted) return;
-      setState(() {
-        _status = status;
-        _statusReceivedAt = DateTime.now();
-        _error = null;
-        _isLoading = false;
-      });
-    } catch (e) {
-      if (!mounted) return;
-      setState(() {
-        _error = e.toString();
-        _isLoading = false;
-      });
-    }
-  }
-
   @override
   Widget build(BuildContext context) {
     return Column(
@@ -69,12 +12,7 @@ class _StatusScreenState extends State<StatusScreen> {
       children: [
         Text('Status', style: Theme.of(context).textTheme.headlineMedium),
         const SizedBox(height: 20),
-        ArpScannerCard(
-          status: _status,
-          statusReceivedAt: _statusReceivedAt,
-          error: _error,
-          isLoading: _isLoading,
-        ),
+        const ArpScannerCard(),
       ],
     );
   }