fix: 修复多个金融计算bug

1. 卖出时累计成本计算：改为按比例减少成本，而非用卖出金额抵扣
2. 夏普比率计算：收益率从百分比转为小数后计算，修正年化公式
3. 最大回撤初始值：使用首条记录的净值作为初始peak，而非硬编码1.0

AssetManager.Services/PortfolioNavService.cs

@@ -182,27 +182,54 @@ public class PortfolioNavService : IPortfolioNavService
             .OrderByDescending(n => n.NavDate)
             .FirstAsync();
 
-        // 计算累计投入成本（从交易记录汇总）
+        // 计算累计投入成本（从交易记录汇总，卖出时按比例减少成本）
         var transactions = await _db.Queryable<Transaction>()
             .Where(t => t.PortfolioId == portfolioId && t.TransactionTime.Date <= today)
+            .OrderBy(t => t.TransactionTime)
             .ToListAsync();
 
         decimal totalCost = 0;
+        var holdingsCost = new Dictionary<string, decimal>(); // 每个标的的累计成本
+        
         foreach (var tx in transactions)
         {
+            if (tx.StockCode == null) continue;
+            
             if (tx.Type == "buy")
             {
                 decimal txAmount = tx.TotalAmount;
                 decimal txAmountInTarget = await _exchangeRateService.ConvertAmountAsync(
                     txAmount, tx.Currency, targetCurrency);
                 totalCost += txAmountInTarget;
+                
+                // 更新该标的的累计成本
+                if (holdingsCost.ContainsKey(tx.StockCode))
+                {
+                    holdingsCost[tx.StockCode] += txAmountInTarget;
+                }
+                else
+                {
+                    holdingsCost[tx.StockCode] = txAmountInTarget;
+                }
             }
             else if (tx.Type == "sell")
             {
-                decimal txAmount = tx.TotalAmount;
-                decimal txAmountInTarget = await _exchangeRateService.ConvertAmountAsync(
-                    txAmount, tx.Currency, targetCurrency);
-                totalCost -= txAmountInTarget;
+                // 卖出时按比例减少该标的的累计成本
+                if (holdingsCost.ContainsKey(tx.StockCode) && tx.Amount > 0)
+                {
+                    // 需要知道当时该标的的总数量来计算比例
+                    // 从 Position 表获取当前持仓数量（不精确，但作为近似）
+                    var position = positions.FirstOrDefault(p => p.StockCode == tx.StockCode);
+                    if (position != null && position.Shares > 0)
+                    {
+                        // 近似：用当前持仓数量 + 卖出数量 作为原来数量
+                        decimal originalShares = position.Shares + tx.Amount;
+                        decimal soldRatio = tx.Amount / originalShares;
+                        decimal costToReduce = holdingsCost[tx.StockCode] * soldRatio;
+                        holdingsCost[tx.StockCode] -= costToReduce;
+                        totalCost -= costToReduce;
+                    }
+                }
             }
         }
 
@@ -381,10 +408,9 @@ public class PortfolioNavService : IPortfolioNavService
                             holdings[tx.StockCode] = (remainingShares, remainingCost, existing.currency, existing.assetType);
                         }
 
-                        // 减少累计投入成本
-                        decimal txAmountInTarget = await _exchangeRateService.ConvertAmountAsync(
-                            tx.TotalAmount, tx.Currency, targetCurrency);
-                        cumulativeCost -= txAmountInTarget;
+                        // 按比例减少累计投入成本（关键修复：不再用卖出金额）
+                        decimal costToReduce = cumulativeCost * (decimal)soldRatio;
+                        cumulativeCost -= costToReduce;
                     }
                 }
             }
@@ -560,10 +586,11 @@ public class PortfolioNavService : IPortfolioNavService
         var maxReturn = returns.Any() ? returns.Max() : 0;
         var minReturn = returns.Any() ? returns.Min() : 0;
 
-        // 计算最大回撤
+        // 计算最大回撤（修复：用第一条记录的 Nav 作为初始 peak）
         double maxDrawdown = 0;
-        double peak = 1.0;
-        foreach (var item in history.OrderBy(h => h.NavDate))
+        var orderedHistory = history.OrderBy(h => h.NavDate).ToList();
+        double peak = (double)orderedHistory.First().Nav;
+        foreach (var item in orderedHistory)
         {
             var nav = (double)item.Nav;
             if (nav > peak) peak = nav;
@@ -571,15 +598,21 @@ public class PortfolioNavService : IPortfolioNavService
             if (drawdown > maxDrawdown) maxDrawdown = drawdown;
         }
 
-        // 计算夏普比率（简化版，假设无风险利率=3%年化）
+        // 计算夏普比率（修复：收益率从百分比转为小数）
         double sharpeRatio = 0;
         double volatility = 0;
         if (returns.Any())
         {
-            var avgReturn = returns.Average();
-            var stdDev = Math.Sqrt(returns.Sum(r => Math.Pow(r - avgReturn, 2)) / returns.Count);
-            volatility = stdDev * Math.Sqrt(252); // 年化波动率
-            sharpeRatio = stdDev > 0 ? (avgReturn * 252 - 3) / (stdDev * Math.Sqrt(252)) : 0;
+            // 收益率从百分比形式转换为小数形式（如 0.5% -> 0.005）
+            var decimalReturns = returns.Select(r => r / 100.0).ToList();
+            var avgReturn = decimalReturns.Average();
+            var stdDev = Math.Sqrt(decimalReturns.Sum(r => Math.Pow(r - avgReturn, 2)) / decimalReturns.Count);
+            
+            // 年化
+            double annualizedReturn = avgReturn * 252;
+            double annualizedVol = stdDev * Math.Sqrt(252);
+            volatility = annualizedVol * 100; // 转回百分比形式显示
+            sharpeRatio = annualizedVol > 0 ? (annualizedReturn - 0.03) / annualizedVol : 0; // 3% 无风险利率
         }
 
         // 总收益率