fix：参考金值，研听，研值工程通用通用工程公共能力，重点对比网络请求、登录认证、状态管理、本地存储、WebView、推送、版本更新、一键登录、主题/UI 等模块，确定后续统一整改方案

lib/features/assets/data/asset_models.dart

@@ -51,6 +51,45 @@ class GoldAssetHolding {
   final HoldingStatus status;
 
   double materialValue(double spotPrice) => weightGram * purity * spotPrice;
+
+  Map<String, Object?> toJson() => {
+    'id': id,
+    'name': name,
+    'metal': metal.name,
+    'category': category.name,
+    'purity': purity,
+    'purityLabel': purityLabel,
+    'weightGram': weightGram,
+    'costAmount': costAmount,
+    'purchaseDate': purchaseDate?.toIso8601String(),
+    'channel': channel,
+    'note': note,
+    'createdAt': createdAt.toIso8601String(),
+    'updatedAt': updatedAt.toIso8601String(),
+    'status': status.name,
+  };
+
+  factory GoldAssetHolding.fromJson(Map<String, Object?> json) {
+    return GoldAssetHolding(
+      id: json['id']! as String,
+      name: json['name']! as String,
+      metal: MetalType.values.byName(json['metal']! as String),
+      category: AssetCategory.values.byName(json['category']! as String),
+      purity: (json['purity']! as num).toDouble(),
+      purityLabel: json['purityLabel']! as String,
+      weightGram: (json['weightGram']! as num).toDouble(),
+      costAmount: (json['costAmount'] as num?)?.toDouble(),
+      purchaseDate: switch (json['purchaseDate']) {
+        final String value when value.isNotEmpty => DateTime.parse(value),
+        _ => null,
+      },
+      channel: json['channel'] as String?,
+      note: json['note'] as String?,
+      createdAt: DateTime.parse(json['createdAt']! as String),
+      updatedAt: DateTime.parse(json['updatedAt']! as String),
+      status: HoldingStatus.values.byName(json['status']! as String),
+    );
+  }
 }
 
 class HoldingValuation {