Pause polling when screens are off-route or app is backgrounded

PolledValue gains pause()/resume() methods. ScannersStatusCard and
NotificationsList now implement RouteAware (didPushNext/didPopNext)
and WidgetsBindingObserver to stop API calls and tick timers when
the widget is not visible, resuming with an immediate fetch on return.

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

frontend/lib/home/notifications_list.dart

@@ -37,7 +37,8 @@ class NotificationsList extends StatefulWidget {
   State<NotificationsList> createState() => _NotificationsListState();
 }
 
-class _NotificationsListState extends State<NotificationsList> with RouteAware {
+class _NotificationsListState extends State<NotificationsList>
+    with RouteAware, WidgetsBindingObserver {
   _NotificationFilter _filter = _NotificationFilter.newOnly;
   Timer? _notificationTimer;
 
@@ -51,11 +52,9 @@ class _NotificationsListState extends State<NotificationsList> with RouteAware {
   @override
   void initState() {
     super.initState();
+    WidgetsBinding.instance.addObserver(this);
     _fetchPage(0);
-    _notificationTimer = Timer.periodic(
-      const Duration(minutes: 1),
-      (_) => _fetchPage(_currentPage),
-    );
+    _startTimer();
   }
 
   @override
@@ -67,13 +66,40 @@ class _NotificationsListState extends State<NotificationsList> with RouteAware {
     }
   }
 
+  @override
+  void didPushNext() {
+    _notificationTimer?.cancel();
+    _notificationTimer = null;
+  }
+
   @override
   void didPopNext() {
     _fetchPage(_currentPage);
+    _startTimer();
+  }
+
+  @override
+  void didChangeAppLifecycleState(AppLifecycleState state) {
+    if (state == AppLifecycleState.paused) {
+      _notificationTimer?.cancel();
+      _notificationTimer = null;
+    } else if (state == AppLifecycleState.resumed) {
+      _fetchPage(_currentPage);
+      _startTimer();
+    }
+  }
+
+  void _startTimer() {
+    _notificationTimer?.cancel();
+    _notificationTimer = Timer.periodic(
+      const Duration(minutes: 1),
+      (_) => _fetchPage(_currentPage),
+    );
   }
 
   @override
   void dispose() {
+    WidgetsBinding.instance.removeObserver(this);
     routeObserver.unsubscribe(this);
     _notificationTimer?.cancel();
     _fetchToken?.cancel();

frontend/lib/utils/polled_value.dart

@@ -13,12 +13,14 @@ class PolledValue<T> extends ChangeNotifier {
     required Duration pollInterval,
     required Duration staleErrorAfter,
   }) : _fetch = fetch,
+       _pollInterval = pollInterval,
        _staleErrorAfter = staleErrorAfter {
     _load();
     _pollTimer = Timer.periodic(pollInterval, (_) => _load());
   }
 
   final Future<T> Function({CancelToken? cancelToken}) _fetch;
+  final Duration _pollInterval;
   final Duration _staleErrorAfter;
   Timer? _pollTimer;
   CancelToken? _cancelToken;
@@ -66,6 +68,18 @@ class PolledValue<T> extends ChangeNotifier {
     }
   }
 
+  void pause() {
+    _pollTimer?.cancel();
+    _pollTimer = null;
+    _cancelToken?.cancel();
+  }
+
+  void resume() {
+    if (_disposed) return;
+    _load();
+    _pollTimer = Timer.periodic(_pollInterval, (_) => _load());
+  }
+
   @override
   void dispose() {
     _disposed = true;

frontend/lib/widgets/scanners_status_card.dart

@@ -5,6 +5,7 @@ import 'package:go_router/go_router.dart';
 
 import '../model/arp_scanner_status.dart';
 import '../model/mdns_scanner_status.dart';
+import '../navigation.dart';
 import '../utils/duration_formatter.dart';
 import '../utils/oott_api.dart';
 import '../utils/polled_value.dart';
@@ -17,7 +18,8 @@ class ScannersStatusCard extends StatefulWidget {
   State<ScannersStatusCard> createState() => _ScannersStatusCardState();
 }
 
-class _ScannersStatusCardState extends State<ScannersStatusCard> {
+class _ScannersStatusCardState extends State<ScannersStatusCard>
+    with RouteAware, WidgetsBindingObserver {
   late final PolledValue<ArpScannerStatus> _arp;
   late final PolledValue<MdnsScannerStatus> _mdns;
   Timer? _tickTimer;
@@ -25,6 +27,7 @@ class _ScannersStatusCardState extends State<ScannersStatusCard> {
   @override
   void initState() {
     super.initState();
+    WidgetsBinding.instance.addObserver(this);
     _arp = PolledValue<ArpScannerStatus>(
       fetch: ({cancelToken}) =>
           BackendAPI.instance.getArpScannerStatus(cancelToken: cancelToken),
@@ -42,8 +45,53 @@ class _ScannersStatusCardState extends State<ScannersStatusCard> {
     });
   }
 
+  @override
+  void didChangeDependencies() {
+    super.didChangeDependencies();
+    final route = ModalRoute.of(context);
+    if (route is ModalRoute<void>) {
+      routeObserver.subscribe(this, route);
+    }
+  }
+
+  @override
+  void didPushNext() {
+    _pausePolling();
+  }
+
+  @override
+  void didPopNext() {
+    _resumePolling();
+  }
+
+  @override
+  void didChangeAppLifecycleState(AppLifecycleState state) {
+    if (state == AppLifecycleState.paused) {
+      _pausePolling();
+    } else if (state == AppLifecycleState.resumed) {
+      _resumePolling();
+    }
+  }
+
+  void _pausePolling() {
+    _arp.pause();
+    _mdns.pause();
+    _tickTimer?.cancel();
+    _tickTimer = null;
+  }
+
+  void _resumePolling() {
+    _arp.resume();
+    _mdns.resume();
+    _tickTimer ??= Timer.periodic(const Duration(seconds: 1), (_) {
+      if (mounted) setState(() {});
+    });
+  }
+
   @override
   void dispose() {
+    WidgetsBinding.instance.removeObserver(this);
+    routeObserver.unsubscribe(this);
     _tickTimer?.cancel();
     _arp.dispose();
     _mdns.dispose();