fix: 修复资金账户余额显示0.00的问题

主要修改:
1. asset_page.dart - _FundAccountCard组件
   - 添加fallback逻辑：优先使用fund.balance，如果为null则使用overview.fundBalance
   - 确保即使fund数据未加载也能显示余额

2. asset_page.dart - initState方法
   - 强制刷新数据（force: true），不使用缓存
   - 确保每次进入页面都加载最新数据

3. 添加账户体系设计文档
   - ACCOUNT_SYSTEM_DESIGN.md - 完整的账户体系设计
   - check_flutter_data.md - 问题诊断步骤

修复后效果:
- ✅ 资金账户余额正确显示 (15500 USDT)
- ✅ 数据加载更可靠
- ✅ 页面刷新时强制更新数据

问题原因:
- fund数据可能未加载或为null
- 添加fallback到overview.fundBalance
- 强制刷新确保数据最新

test_flutter_asset.sh

@@ -0,0 +1,89 @@
+#!/bin/bash
+
+BASE_URL="http://localhost:5010"
+
+echo "================================"
+echo "测试Flutter资产页面API调用"
+echo "================================"
+
+# 1. 用户登录
+echo -e "\n步骤1: 用户登录..."
+LOGIN_RESPONSE=$(curl -s -X POST "$BASE_URL/api/user/login" \
+  -H "Content-Type: application/json" \
+  -d '{"username":"abcd","password":"abcd123"}')
+
+TOKEN=$(echo $LOGIN_RESPONSE | python3 -c "
+import sys, json
+data = json.load(sys.stdin)
+if data.get('success'):
+    print(data['data']['token'])
+" 2>/dev/null)
+
+if [ -z "$TOKEN" ]; then
+    echo "❌ 登录失败"
+    exit 1
+fi
+
+echo "✅ 登录成功"
+
+# 2. 测试资产总览（Flutter首页调用）
+echo -e "\n================================"
+echo "测试1: /api/asset/overview (资产总览)"
+echo "================================"
+OVERVIEW=$(curl -s "$BASE_URL/api/asset/overview" \
+  -H "Authorization: Bearer $TOKEN")
+echo "$OVERVIEW" | python3 -m json.tool
+
+# 3. 测试资金账户（Flutter资金账户Tab调用）
+echo -e "\n================================"
+echo "测试2: /api/asset/fund (资金账户)"
+echo "================================"
+FUND=$(curl -s "$BASE_URL/api/asset/fund" \
+  -H "Authorization: Bearer $TOKEN")
+echo "$FUND" | python3 -m json.tool
+
+# 4. 测试交易账户（Flutter交易账户Tab调用）
+echo -e "\n================================"
+echo "测试3: /api/asset/trade (交易账户)"
+echo "================================"
+TRADE=$(curl -s "$BASE_URL/api/asset/trade" \
+  -H "Authorization: Bearer $TOKEN")
+echo "$TRADE" | python3 -m json.tool
+
+# 5. 分析数据结构
+echo -e "\n================================"
+echo "数据分析"
+echo "================================"
+echo "$FUND" | python3 -c "
+import sys, json
+data = json.load(sys.stdin)
+if data.get('success') and data.get('data'):
+    fund = data['data'].get('fund', {})
+    print('资金账户数据:')
+    print(f'  - balance: {fund.get(\"balance\")}')
+    print(f'  - frozen: {fund.get(\"frozen\")}')
+    print(f'  - total_deposit: {fund.get(\"totalDeposit\")}')
+    print(f'  - total_withdraw: {fund.get(\"totalWithdraw\")}')
+    
+    if fund.get('balance') == 0:
+        print('\n⚠️  余额为0，但数据库中应该有15500')
+    else:
+        print(f'\n✅ 余额正确: {fund.get(\"balance\")}')
+"
+
+echo "$TRADE" | python3 -c "
+import sys, json
+data = json.load(sys.stdin)
+if data.get('success') and data.get('data'):
+    positions = data['data'].get('positions', [])
+    print(f'\n交易账户持仓数: {len(positions)}')
+    if positions:
+        for pos in positions:
+            print(f'  - {pos.get(\"coinCode\")}: {pos.get(\"quantity\")} (价值: {pos.get(\"value\")} USDT)')
+    else:
+        print('  暂无持仓')
+"
+
+echo -e "\n================================"
+echo "测试完成"
+echo "================================"