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/model/active_scanner_status.dart

@@ -0,0 +1,27 @@
+/// Status of an active scanner (one that runs on an interval, e.g. ARP, SNMP).
+/// Mirrors the backend's `ActiveSnapshot` / `ActiveScannerStatusResponse`.
+class ActiveScannerStatus {
+  final bool isRunning;
+  final double? runningForSeconds;
+  final double? nextRunInSeconds;
+  final int? lastScanDevicesSeen;
+  final double? lastScanSecondsAgo;
+
+  const ActiveScannerStatus({
+    required this.isRunning,
+    this.runningForSeconds,
+    this.nextRunInSeconds,
+    this.lastScanDevicesSeen,
+    this.lastScanSecondsAgo,
+  });
+
+  factory ActiveScannerStatus.fromJson(Map<String, dynamic> json) {
+    return ActiveScannerStatus(
+      isRunning: json['is_running'] as bool,
+      runningForSeconds: (json['running_for_seconds'] as num?)?.toDouble(),
+      nextRunInSeconds: (json['next_run_in_seconds'] as num?)?.toDouble(),
+      lastScanDevicesSeen: (json['last_scan_devices_seen'] as num?)?.toInt(),
+      lastScanSecondsAgo: (json['last_scan_seconds_ago'] as num?)?.toDouble(),
+    );
+  }
+}