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/api/oott_api_scanners.dart

@@ -1,40 +1,45 @@
 part of '../oott_api.dart';
 
-/// Per-scanner status endpoints. Every scanner exposes the same
-/// `/<scanner>_scanner/status` shape, so each getter just decodes its model.
+/// Per-scanner status endpoints. Active scanners (ARP, SNMP) share the
+/// [ActiveScannerStatus] shape; passive scanners (mDNS, SSDP, DHCP) share the
+/// [PassiveScannerStatus] shape. Each getter just decodes its model.
 extension ScannerApi on BackendAPI {
-  Future<ArpScannerStatus> getArpScannerStatus({CancelToken? cancelToken}) =>
+  Future<ActiveScannerStatus> getArpScannerStatus({CancelToken? cancelToken}) =>
       _getModel(
         '/arp_scanner/status',
-        ArpScannerStatus.fromJson,
+        ActiveScannerStatus.fromJson,
         cancelToken: cancelToken,
       );
 
-  Future<MdnsScannerStatus> getMdnsScannerStatus({CancelToken? cancelToken}) =>
-      _getModel(
-        '/mdns_scanner/status',
-        MdnsScannerStatus.fromJson,
-        cancelToken: cancelToken,
-      );
+  Future<PassiveScannerStatus> getMdnsScannerStatus({
+    CancelToken? cancelToken,
+  }) => _getModel(
+    '/mdns_scanner/status',
+    PassiveScannerStatus.fromJson,
+    cancelToken: cancelToken,
+  );
 
-  Future<SsdpScannerStatus> getSsdpScannerStatus({CancelToken? cancelToken}) =>
-      _getModel(
-        '/ssdp_scanner/status',
-        SsdpScannerStatus.fromJson,
-        cancelToken: cancelToken,
-      );
+  Future<PassiveScannerStatus> getSsdpScannerStatus({
+    CancelToken? cancelToken,
+  }) => _getModel(
+    '/ssdp_scanner/status',
+    PassiveScannerStatus.fromJson,
+    cancelToken: cancelToken,
+  );
 
-  Future<DhcpScannerStatus> getDhcpScannerStatus({CancelToken? cancelToken}) =>
-      _getModel(
-        '/dhcp_scanner/status',
-        DhcpScannerStatus.fromJson,
-        cancelToken: cancelToken,
-      );
+  Future<PassiveScannerStatus> getDhcpScannerStatus({
+    CancelToken? cancelToken,
+  }) => _getModel(
+    '/dhcp_scanner/status',
+    PassiveScannerStatus.fromJson,
+    cancelToken: cancelToken,
+  );
 
-  Future<SnmpScannerStatus> getSnmpScannerStatus({CancelToken? cancelToken}) =>
-      _getModel(
-        '/snmp_scanner/status',
-        SnmpScannerStatus.fromJson,
-        cancelToken: cancelToken,
-      );
+  Future<ActiveScannerStatus> getSnmpScannerStatus({
+    CancelToken? cancelToken,
+  }) => _getModel(
+    '/snmp_scanner/status',
+    ActiveScannerStatus.fromJson,
+    cancelToken: cancelToken,
+  );
 }