fix: stop-loss on spread reversal + current spread sign (#3)

- Add explicit stop-loss when diffPct < -0.02 (价差反转,止损平仓)
  instead of relying on the convergence threshold to catch reversals
- Dashboard currentSpread now direction-aware for HL→BG positions
- Trade detail modal with clickable rows (previous commit partial)

Before: reversals would exit via  with wrong reason
  '价差收敛'. After: dedicated < -0.02 check with correct reason.

trader.go

@@ -454,6 +454,12 @@ func (t *Trader) checkExit(pos *ArbPosition, bgP, hlP, diffPct float64, notifier
 		exitReason = "价差收敛,止盈平仓"
 	}
 
+	// Stop-loss: spread reversed (went negative)
+	if diffPct < -0.02 {
+		shouldExit = true
+		exitReason = "价差反转,止损平仓"
+	}
+
 	if elapsed > 30*time.Minute {
 		shouldExit = true
 		exitReason = "超时平仓"