fix(trader): harden API calls with timeouts, strict balance parsing, error context

- binance/bybit/gate: SDK default http.DefaultClient has no timeout; use a
  dedicated 30s-timeout client so a hung connection cannot stall the loop
- bybit: stop mutating http.DefaultClient.Transport, which leaked the
  referer header into every other HTTP request in the process
- add types.ParseFloatField: empty exchange fields stay zero, but malformed
  numeric values now surface as errors instead of silently becoming zero
  balances (applied to GetBalance across 8 exchanges)
- wrap order/market-data errors in auto_trader_orders and okx cancel paths
  with symbol context; log per-order cancel failures in okx CancelAllOrders

trader/bitget/trader_account.go

@@ -4,6 +4,7 @@ import (
 	"encoding/json"
 	"fmt"
 	"nofx/logger"
+	"nofx/trader/types"
 	"strconv"
 	"strings"
 	"time"
@@ -43,9 +44,15 @@ func (t *BitgetTrader) GetBalance() (map[string]interface{}, error) {
 	var totalEquity, availableBalance, unrealizedPnL float64
 	for _, acc := range accounts {
 		if acc.MarginCoin == "USDT" {
-			totalEquity, _ = strconv.ParseFloat(acc.AccountEquity, 64)
-			availableBalance, _ = strconv.ParseFloat(acc.Available, 64)
-			unrealizedPnL, _ = strconv.ParseFloat(acc.UnrealizedPL, 64)
+			if totalEquity, err = types.ParseFloatField("accountEquity", acc.AccountEquity); err != nil {
+				return nil, err
+			}
+			if availableBalance, err = types.ParseFloatField("available", acc.Available); err != nil {
+				return nil, err
+			}
+			if unrealizedPnL, err = types.ParseFloatField("unrealizedPL", acc.UnrealizedPL); err != nil {
+				return nil, err
+			}
 			logger.Infof("✓ [Bitget] Balance: equity=%.2f, available=%.2f", totalEquity, availableBalance)
 			break
 		}