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

trader.go

@@ -183,6 +183,35 @@ func NewTrader(cfg *Config, database *db.DB) *Trader {
 	// Restore open positions from DB on restart
 	if database != nil {
 		t.restoreOpenPositions()
+
+		// Load historical closed trades for PnL stats (so total PnL survives restart)
+		if closed, err := database.GetAllClosedTrades(); err == nil {
+			for i := range closed {
+				dbTr := &closed[i]
+				pnlPct := safeFloat(dbTr.NetPnl)
+				pnlUSD := 2 * dbTr.AmountUSD * pnlPct / 100
+				closedAt := time.Time{}
+				if dbTr.ClosedAt != nil {
+					closedAt = *dbTr.ClosedAt
+				}
+				record := TradeRecord{
+					Coin:        dbTr.Coin,
+					Direction:   dbTr.Direction,
+					EntrySpread: safeFloat(dbTr.EntrySpread),
+					ExitSpread:  safeFloat(dbTr.ExitSpread),
+					PnlPct:      pnlPct,
+					PnlUSD:      pnlUSD,
+					Convergence: safeStr(dbTr.Convergence),
+					Reason:      safeStr(dbTr.ExitReason),
+					Duration:    closedAt.Sub(dbTr.OpenedAt).Round(time.Second).String(),
+					OpenedAt:    dbTr.OpenedAt,
+					ClosedAt:    closedAt,
+					ScaleLevels: dbTr.ScaleCount,
+					AmountUSD:   dbTr.AmountUSD,
+				}
+				t.closedTrades = append(t.closedTrades, record)
+			}
+		}
 		// Load historical closed trade stats for convergence display
 		if c, d, f, tot, err := database.GetClosedStats(); err == nil {
 			t.dbConverged, t.dbDiverged, t.dbFlat, t.dbTotal = c, d, f, tot
@@ -1282,4 +1311,20 @@ func (t *Trader) RemoveBlacklist(coin string) {
 	defer t.mu.Unlock()
 	delete(t.blacklist, coin)
 	log.Printf("[Trader] ✅ %s: Removed from blacklist", coin)
+}
+
+// safeFloat returns 0 for nil float64 pointers (DB nullable fields).
+func safeFloat(f *float64) float64 {
+	if f == nil {
+		return 0
+	}
+	return *f
+}
+
+// safeStr returns empty string for nil string pointers (DB nullable fields).
+func safeStr(s *string) string {
+	if s == nil {
+		return ""
+	}
+	return *s
 }