fix: 重启后总PnL回到0的问题 + 前端PnL图表公式统一

- trader.go: NewTrader 启动时从DB GetAllClosedTrades() 加载所有已平仓交易到 closedTrades
- trader.go: 新增 safeFloat/safeStr 辅助函数处理DB空指针
- db/trade_repo.go: 新增 GetAllClosedTrades() 方法
- frontend: PnL图表标题和曲线改用 2*AmountUSD*NetPnl/100 公式，与后端Stats一致

db/trade_repo.go

@@ -260,6 +260,20 @@ func (d *DB) GetScalePrices(tradeID int64) (longPrices, shortPrices []float64, e
 	return longPrices, shortPrices, rows.Err()
 }
 
+// GetAllClosedTrades returns all closed trades for PnL history restoration.
+func (d *DB) GetAllClosedTrades() ([]TradeRecord, error) {
+	rows, err := d.Query(`SELECT id, coin, direction, status, entry_spread, exit_spread,
+		long_exchange, short_exchange, long_entry, long_exit, short_entry, short_exit,
+		long_pnl, short_pnl, fee_entry, fee_exit, net_pnl,
+		amount_usd, scale_count, exit_reason, convergence, opened_at, closed_at
+		FROM trades WHERE status='closed' ORDER BY id`)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+	return scanTrades(rows)
+}
+
 // GetClosedStats returns convergence counts from the database.
 func (d *DB) GetClosedStats() (converged, diverged, flat, total int, err error) {
 	if err = d.QueryRow("SELECT COUNT(*) FROM trades WHERE status='closed'").Scan(&total); err != nil {