NinjaTrader CSV: correct net PnL and currency parsing

NinjaTrader’s Profit column is already net of costs for that row. The performance CSV importer used to add commission again on top, which inflated trade PnL. It now leaves profit as-is and keeps commission as its own field, matching how NinjaTrader’s cumulative net profit steps work.

Currency parsing also moved into a shared helper that handles US and European number styles, parentheses for negatives, and thousand separators more consistently, so fewer rows silently land at zero.