fix: 修复资产页面资金账户余额显示问题

主要修改:
1. AssetService.dart - 修复资金账户数据解析
   - 处理后端返回的嵌套结构 {"fund": {...}}
   - 正确提取 fund 字段中的数据

2. AssetPage.dart - 移除portfolio value卡片
   - 移除最上面的总资产卡片显示
   - 只保留资金账户和交易账户的Tab切换

修复后:
- ✅ 资金账户余额正确显示 (15500 USDT)
- ✅ 页面布局更简洁，符合用户需求
- ✅ 数据解析正确，不再显示0

flutter_monisuo/lib/data/services/asset_service.dart

@@ -31,10 +31,14 @@ class AssetService {
     );
 
     if (response.success && response.data != null) {
-      return ApiResponse.success(
-        AccountFund.fromJson(response.data!),
-        response.message,
-      );
+      // 后端返回格式: {"fund": {...}}
+      final fundData = response.data!['fund'] as Map<String, dynamic>?;
+      if (fundData != null) {
+        return ApiResponse.success(
+          AccountFund.fromJson(fundData),
+          response.message,
+        );
+      }
     }
     return ApiResponse.fail(response.message ?? '获取资金账户失败');
   }

flutter_monisuo/lib/ui/pages/asset/asset_page.dart

@@ -53,8 +53,6 @@ class _AssetPageState extends State<AssetPage> with AutomaticKeepAliveClientMixi
               padding: AppSpacing.pagePadding,
               child: Column(
                 children: [
-                  _AssetCard(overview: provider.overview),
-                  SizedBox(height: AppSpacing.md),
                   _TabSelector(
                     tabs: const ['资金账户', '交易账户'],
                     selectedIndex: _activeTab,