Consolidate frontend scanner, pagination, and tick-timer duplication

Collapse the five scanner status models into two shared shapes,
ActiveScannerStatus and PassiveScannerStatus, mirroring the backend's
active/passive vocabulary. Replace the five near-identical per-scanner
detail card files with a single scanner_status_cards.dart (two shared
resolvers plus a config list), and rebuild the combined home card to
iterate a list of scanners with two shape resolvers instead of five
copy-pasted resolve methods.

Extract two reusable mixins:
- PeriodicRebuild: the shared once-a-second "rebuild to refresh elapsed
  text" timer used by the scanner cards and the stale indicator.
- PaginatedListState: the shared pagination state, page-size/page-count
  getters, cancel-token-aware fetch orchestration, and disposal used by
  the device and notification lists.

No behaviour change; ~900 lines removed. Tests and analyzer pass.

frontend/lib/utils/periodic_rebuild.dart

@@ -0,0 +1,33 @@
+import 'dart:async';
+
+import 'package:flutter/widgets.dart';
+
+/// Mixin for [State]s that display continuously-updating elapsed-time text
+/// (e.g. "Running for 3m 12s"). It runs a once-per-second timer that calls
+/// [setState] so those labels stay current, and cancels it automatically on
+/// dispose.
+///
+/// Call [startRebuildTicker] from `initState` (or when becoming visible) and
+/// [stopRebuildTicker] when the widget is paused/hidden. Both are idempotent.
+mixin PeriodicRebuild<T extends StatefulWidget> on State<T> {
+  Timer? _rebuildTicker;
+
+  /// Starts the per-second rebuild ticker if it isn't already running.
+  void startRebuildTicker() {
+    _rebuildTicker ??= Timer.periodic(const Duration(seconds: 1), (_) {
+      if (mounted) setState(() {});
+    });
+  }
+
+  /// Stops the per-second rebuild ticker.
+  void stopRebuildTicker() {
+    _rebuildTicker?.cancel();
+    _rebuildTicker = null;
+  }
+
+  @override
+  void dispose() {
+    stopRebuildTicker();
+    super.dispose();
+  }
+}