feat: add pending orders (SL/TP) display on chart

- Add GetOpenOrders method to Trader interface
- Implement for Binance (legacy + Algo), Bybit, Hyperliquid
- Add stub implementations for OKX, Bitget, Aster, Lighter
- Add /api/open-orders endpoint
- Display price lines for SL (red) and TP (green) orders
- Refresh open orders every 60 seconds (separate from 5s kline refresh)

trader/okx_trader.go

@@ -1387,3 +1387,9 @@ func (t *OKXTrader) GetClosedPnL(startTime time.Time, limit int) ([]ClosedPnLRec
 
 	return records, nil
 }
+
+// GetOpenOrders gets all open/pending orders for a symbol
+func (t *OKXTrader) GetOpenOrders(symbol string) ([]OpenOrder, error) {
+	// TODO: Implement OKX open orders
+	return []OpenOrder{}, nil
+}