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/aster/trader_account.go

@@ -36,14 +36,21 @@ func (t *AsterTrader) GetBalance() (map[string]interface{}, error) {
 			foundUSDT = true
 
 			// Parse Aster fields (reference: https://github.com/asterdex/api-docs)
+			var parseErr error
 			if avail, ok := bal["availableBalance"].(string); ok {
-				availableBalance, _ = strconv.ParseFloat(avail, 64)
+				if availableBalance, parseErr = types.ParseFloatField("availableBalance", avail); parseErr != nil {
+					return nil, parseErr
+				}
 			}
 			if unpnl, ok := bal["crossUnPnl"].(string); ok {
-				crossUnPnl, _ = strconv.ParseFloat(unpnl, 64)
+				if crossUnPnl, parseErr = types.ParseFloatField("crossUnPnl", unpnl); parseErr != nil {
+					return nil, parseErr
+				}
 			}
 			if cwb, ok := bal["crossWalletBalance"].(string); ok {
-				crossWalletBalance, _ = strconv.ParseFloat(cwb, 64)
+				if crossWalletBalance, parseErr = types.ParseFloatField("crossWalletBalance", cwb); parseErr != nil {
+					return nil, parseErr
+				}
 			}
 			break
 		}