feat(binance): Add Binance Futures Testnet support

[JordanzhaoD]

📝 Description | 描述

Add Binance Futures Testnet support to enable safe strategy testing without real funds.
添加币安合约测试网支持，允许在无需真实资金的情况下安全测试策略。

🎯 Type of Change

• ✨ New feature

🔗 Related Issues

• Related to PR feat(okx/hyperliquid): add Testnet/Simulated trading support #1317 (OKX/Hyperliquid testnet support)

📋 Problem Statement

Currently, NOFX supports testnet for OKX/Hyperliquid (PR #1317), but Binance Futures testnet is NOT supported.

When testnet=1 is set in database, trader/binance_futures.go still uses futures.NewClient() which defaults to mainnet (fapi.binance.com).

Result: Testnet API key fails with -2015: Invalid API-key because requests go to mainnet instead of testnet.binancefuture.com.

目前NOFX支持OKX/Hyperliquid测试网（PR #1317），但不支持币安合约测试网。

当数据库设置testnet=1时，trader/binance_futures.go仍使用默认正式网(fapi.binance.com)。

结果：测试网API Key返回-2015: Invalid API-key错误，因为请求发送到正式网而非testnet.binancefuture.com。

🎯 Proposed Solution

Following PR #1317 pattern:

1. Add Testnet property support in BinanceFuturesTrader
2. When testnet=true, use testnet.binancefuture.com as base URL
3. Update UI to show "Use Testnet" toggle for Binance Futures

参考PR #1317模式：

1. 在BinanceFuturesTrader添加Testnet属性支持
2. 当testnet=true时，使用testnet.binancefuture.com
3. UI添加"使用测试网"开关

📚 Technical Details

Current Code:

func NewFuturesTrader(apiKey, secretKey string, userId string) *FuturesTrader {
    client := futures.NewClient(apiKey, secretKey)  // Always mainnet

Suggested:
func NewFuturesTrader(apiKey, secretKey string, testnet bool, userId string) *FuturesTrader {
    if testnet {
        // Use testnet.binancefuture.com
    }

🔗 Binance Testnet Info

- Base URL: https://testnet.binancefuture.com
- Docs: https://testnet.binancefuture.com/en/futures/BTC_USDT
- Faucet: Available on testnet for getting test funds

✅ Labels