Centralize status polling with last-known-good, stale, and error tiers

The four polling status cards (ARP, mDNS, scanners summary, devices
summary) each rolled their own Timer/_status/_error/_isLoading state and
flipped to a red 'Error' UI on the first failed poll, throwing away
last-known-good data that was still in memory. A momentary backend blip
made every card flash red.

Introduce PolledValue<T>, a ChangeNotifier that owns the periodic timer,
in-flight CancelToken, last value, last error, and a freshness
classification (initialLoading / fresh / stale / error). 'stale' kicks in
on the first failure but keeps the value visible; 'error' only takes over
after staleErrorAfter elapses without a success (30s for the 5s scanner
polls, 3min for the 1min device-summary poll). All four cards now branch
on freshness and show a small live-updating warning icon with a tooltip
('Last updated N ago — <error>') while stale, instead of flipping red.

Also threads CancelToken? through the three polled GET endpoints.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/lib/widgets/polled_stale_indicator.dart

@@ -0,0 +1,54 @@
+import 'dart:async';
+
+import 'package:flutter/material.dart';
+
+import '../utils/duration_formatter.dart';
+import '../utils/polled_value.dart';
+
+class PolledStaleIndicator extends StatefulWidget {
+  const PolledStaleIndicator({super.key, required this.polled});
+
+  final PolledValue polled;
+
+  @override
+  State<PolledStaleIndicator> createState() => _PolledStaleIndicatorState();
+}
+
+class _PolledStaleIndicatorState extends State<PolledStaleIndicator> {
+  Timer? _ticker;
+
+  @override
+  void initState() {
+    super.initState();
+    _ticker = Timer.periodic(const Duration(seconds: 1), (_) {
+      if (mounted) setState(() {});
+    });
+  }
+
+  @override
+  void dispose() {
+    _ticker?.cancel();
+    super.dispose();
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    final lastSuccessAt = widget.polled.lastSuccessAt;
+    final ago = lastSuccessAt != null
+        ? formatSeconds(
+            DateTime.now().difference(lastSuccessAt).inSeconds.toDouble(),
+          )
+        : 'a while';
+    final message = widget.polled.lastErrorMessage != null
+        ? 'Last updated $ago ago — ${widget.polled.lastErrorMessage}'
+        : 'Last updated $ago ago';
+    return Tooltip(
+      message: message,
+      child: Icon(
+        Icons.warning_amber_rounded,
+        size: 14,
+        color: Theme.of(context).colorScheme.onSurfaceVariant,
+      ),
+    );
+  }
+}