Split oott_api.dart into per-domain modules

Break the 372-line frontend API client into focused files under
lib/utils/api/: error mapping (api_error.dart), Dio setup behind a
buildDio factory (dio_config.dart), and device/scanner/notification
endpoints as extensions in part files. Collapse the five near-identical
scanner-status methods via a generic _getModel helper and de-duplicate
the pagination logic via a shared _paginate helper.

The BackendAPI singleton and dioErrorToUserMessage remain importable
from oott_api.dart unchanged, so no call sites are affected.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/lib/utils/api/oott_api_scanners.dart

@@ -0,0 +1,40 @@
+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.
+extension ScannerApi on BackendAPI {
+  Future<ArpScannerStatus> getArpScannerStatus({CancelToken? cancelToken}) =>
+      _getModel(
+        '/arp_scanner/status',
+        ArpScannerStatus.fromJson,
+        cancelToken: cancelToken,
+      );
+
+  Future<MdnsScannerStatus> getMdnsScannerStatus({CancelToken? cancelToken}) =>
+      _getModel(
+        '/mdns_scanner/status',
+        MdnsScannerStatus.fromJson,
+        cancelToken: cancelToken,
+      );
+
+  Future<SsdpScannerStatus> getSsdpScannerStatus({CancelToken? cancelToken}) =>
+      _getModel(
+        '/ssdp_scanner/status',
+        SsdpScannerStatus.fromJson,
+        cancelToken: cancelToken,
+      );
+
+  Future<DhcpScannerStatus> getDhcpScannerStatus({CancelToken? cancelToken}) =>
+      _getModel(
+        '/dhcp_scanner/status',
+        DhcpScannerStatus.fromJson,
+        cancelToken: cancelToken,
+      );
+
+  Future<SnmpScannerStatus> getSnmpScannerStatus({CancelToken? cancelToken}) =>
+      _getModel(
+        '/snmp_scanner/status',
+        SnmpScannerStatus.fromJson,
+        cancelToken: cancelToken,
+      );
+}