fix: backtest equity curve bugs (#39)

- Fix SELL logic: correctly reduce position.quantity after selling (core bug)
  Previously positions were never decremented, causing sold positions to be
  settled again at backtest end, inflating final balance.
- Fix settleRemainingPositions: profitLoss now correctly computes
  settlementValue - cost (was incorrectly using settlementValue.negate())
- Fix max drawdown calculation: use current balance instead of previous
  iteration's runningBalance
- Frontend: add comment noting chart shows cash balance, not total equity

frontend/src/pages/BacktestChart.tsx

@@ -10,6 +10,8 @@ interface BacktestChartProps {
   }[]
 }
 
+// Bug #39 Note: This chart currently displays cash balance (balanceAfter), not total equity.
+// A true equity curve (cash + position value) would require an equityAfter field in the trade records.
 const BacktestChart: React.FC<BacktestChartProps> = ({ trades }) => {
   const { t } = useTranslation()
   const chartRef = useRef<HTMLDivElement>(null)