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

@@ -1,11 +1,13 @@
-class ArpScannerStatus {
+/// 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 ArpScannerStatus({
+  const ActiveScannerStatus({
     required this.isRunning,
     this.runningForSeconds,
     this.nextRunInSeconds,
@@ -13,8 +15,8 @@ class ArpScannerStatus {
     this.lastScanSecondsAgo,
   });
 
-  factory ArpScannerStatus.fromJson(Map<String, dynamic> json) {
-    return ArpScannerStatus(
+  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(),

frontend/lib/model/dhcp_scanner_status.dart

@@ -1,23 +0,0 @@
-class DhcpScannerStatus {
-  final bool isListening;
-  final double? listeningForSeconds;
-  final int devicesSeen;
-  final double? lastDeviceSeenSecondsAgo;
-
-  const DhcpScannerStatus({
-    required this.isListening,
-    this.listeningForSeconds,
-    required this.devicesSeen,
-    this.lastDeviceSeenSecondsAgo,
-  });
-
-  factory DhcpScannerStatus.fromJson(Map<String, dynamic> json) {
-    return DhcpScannerStatus(
-      isListening: json['is_listening'] as bool,
-      listeningForSeconds: (json['listening_for_seconds'] as num?)?.toDouble(),
-      devicesSeen: (json['devices_seen'] as num).toInt(),
-      lastDeviceSeenSecondsAgo: (json['last_device_seen_seconds_ago'] as num?)
-          ?.toDouble(),
-    );
-  }
-}

frontend/lib/model/mdns_scanner_status.dart

@@ -1,23 +0,0 @@
-class MdnsScannerStatus {
-  final bool isListening;
-  final double? listeningForSeconds;
-  final int devicesSeen;
-  final double? lastDeviceSeenSecondsAgo;
-
-  const MdnsScannerStatus({
-    required this.isListening,
-    this.listeningForSeconds,
-    required this.devicesSeen,
-    this.lastDeviceSeenSecondsAgo,
-  });
-
-  factory MdnsScannerStatus.fromJson(Map<String, dynamic> json) {
-    return MdnsScannerStatus(
-      isListening: json['is_listening'] as bool,
-      listeningForSeconds: (json['listening_for_seconds'] as num?)?.toDouble(),
-      devicesSeen: (json['devices_seen'] as num).toInt(),
-      lastDeviceSeenSecondsAgo: (json['last_device_seen_seconds_ago'] as num?)
-          ?.toDouble(),
-    );
-  }
-}

frontend/lib/model/passive_scanner_status.dart

@@ -1,18 +1,20 @@
-class SsdpScannerStatus {
+/// Status of a passive scanner (one that listens continuously, e.g. mDNS, SSDP,
+/// DHCP). Mirrors the backend's `PassiveSnapshot` / `PassiveScannerStatusResponse`.
+class PassiveScannerStatus {
   final bool isListening;
   final double? listeningForSeconds;
   final int devicesSeen;
   final double? lastDeviceSeenSecondsAgo;
 
-  const SsdpScannerStatus({
+  const PassiveScannerStatus({
     required this.isListening,
     this.listeningForSeconds,
     required this.devicesSeen,
     this.lastDeviceSeenSecondsAgo,
   });
 
-  factory SsdpScannerStatus.fromJson(Map<String, dynamic> json) {
-    return SsdpScannerStatus(
+  factory PassiveScannerStatus.fromJson(Map<String, dynamic> json) {
+    return PassiveScannerStatus(
       isListening: json['is_listening'] as bool,
       listeningForSeconds: (json['listening_for_seconds'] as num?)?.toDouble(),
       devicesSeen: (json['devices_seen'] as num).toInt(),

frontend/lib/model/snmp_scanner_status.dart

@@ -1,25 +0,0 @@
-class SnmpScannerStatus {
-  final bool isRunning;
-  final double? runningForSeconds;
-  final double? nextRunInSeconds;
-  final int? lastScanDevicesSeen;
-  final double? lastScanSecondsAgo;
-
-  const SnmpScannerStatus({
-    required this.isRunning,
-    this.runningForSeconds,
-    this.nextRunInSeconds,
-    this.lastScanDevicesSeen,
-    this.lastScanSecondsAgo,
-  });
-
-  factory SnmpScannerStatus.fromJson(Map<String, dynamic> json) {
-    return SnmpScannerStatus(
-      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(),
-    );
-  }
-}