feat(network): add opt-in allow local/private network setting

Add a setting that relaxes the SSRF guard so extensions and built-in network code can reach private/local/loopback targets, for users routing traffic through a local proxy or custom DNS. Disabled by default. Wired end-to-end: Go backend (SetAllowPrivateNetwork toggles isPrivateIP guard), Android/iOS platform bridge, Dart settings model/provider, and a toggle in Download settings.

lib/models/settings.g.dart

@@ -56,6 +56,7 @@ AppSettings _$AppSettingsFromJson(Map<String, dynamic> json) => AppSettings(
       json['autoExportFailedDownloads'] as bool? ?? false,
   downloadNetworkMode: json['downloadNetworkMode'] as String? ?? 'any',
   networkCompatibilityMode: json['networkCompatibilityMode'] as bool? ?? false,
+  allowLocalNetwork: json['allowLocalNetwork'] as bool? ?? false,
   songLinkRegion: json['songLinkRegion'] as String? ?? 'US',
   nativeDownloadWorkerEnabled:
       json['nativeDownloadWorkerEnabled'] as bool? ?? false,
@@ -130,6 +131,7 @@ Map<String, dynamic> _$AppSettingsToJson(
   'autoExportFailedDownloads': instance.autoExportFailedDownloads,
   'downloadNetworkMode': instance.downloadNetworkMode,
   'networkCompatibilityMode': instance.networkCompatibilityMode,
+  'allowLocalNetwork': instance.allowLocalNetwork,
   'songLinkRegion': instance.songLinkRegion,
   'nativeDownloadWorkerEnabled': instance.nativeDownloadWorkerEnabled,
   'localLibraryEnabled': instance.localLibraryEnabled,