fix(ui): make download file deletion explicit in task dialog (#1622) (#1638)

ui/flutter/lib/api/model/downloader_config.dart

@@ -222,7 +222,7 @@ class ExtraConfig {
     this.themeMode = '',
     this.themeColor = 'green',
     this.locale = '',
-    this.lastDeleteTaskKeep = false,
+    this.lastDeleteTaskKeep = true,
     this.defaultDirectDownload = false,
     this.defaultBtClient = true,
     this.notifyWhenNewVersion = true,

ui/flutter/lib/api/model/downloader_config.g.dart

@@ -186,7 +186,7 @@ ExtraConfig _$ExtraConfigFromJson(Map<String, dynamic> json) => ExtraConfig(
   themeMode: json['themeMode'] as String? ?? '',
   themeColor: json['themeColor'] as String? ?? 'green',
   locale: json['locale'] as String? ?? '',
-  lastDeleteTaskKeep: json['lastDeleteTaskKeep'] as bool? ?? false,
+  lastDeleteTaskKeep: json['lastDeleteTaskKeep'] as bool? ?? true,
   defaultDirectDownload: json['defaultDirectDownload'] as bool? ?? false,
   defaultBtClient: json['defaultBtClient'] as bool? ?? true,
   notifyWhenNewVersion: json['notifyWhenNewVersion'] as bool? ?? true,