fix: 修复交易时间解析逻辑

- TransactionDate 和 TransactionTime 正确组合
- 移除 else if 改为独立 if，支持日期+时间
- CreateTransactionAsync 返回实际交易时间而非当前时间

AssetManager.Services/PortfolioService.cs

@@ -657,15 +657,15 @@ public class PortfolioService : IPortfolioService
         {
             if (DateTime.TryParse(request.TransactionDate, out var parsedDate))
             {
-                // 如果只传了日期，时间部分默认用当前时间
-                transactionTime = parsedDate.Date + DateTime.Now.TimeOfDay;
+                transactionTime = parsedDate.Date;
             }
         }
-        else if (!string.IsNullOrEmpty(request.TransactionTime))
+        // 组合时间部分
+        if (!string.IsNullOrEmpty(request.TransactionTime))
         {
-            if (DateTime.TryParse(request.TransactionTime, out var parsedTime))
+            if (TimeSpan.TryParse(request.TransactionTime, out var parsedTime))
             {
-                transactionTime = parsedTime;
+                transactionTime = transactionTime.Date + parsedTime;
             }
         }
 
@@ -821,12 +821,16 @@ public class PortfolioService : IPortfolioService
         request.PortfolioId = portfolioId;
         var response = await CreateTransaction(request, userId);
         
+        // 使用实际交易时间
+        var transactionDate = request.TransactionDate ?? DateTime.Now.ToString("yyyy-MM-dd");
+        var transactionTime = request.TransactionTime ?? DateTime.Now.ToString("HH:mm");
+        
         return new TransactionItem
         {
             Id = response.Id,
             PortfolioId = portfolioId,
-            Date = DateTime.Now.ToString("yyyy-MM-dd"),
-            Time = DateTime.Now.ToString("HH:mm:ss"),
+            Date = transactionDate,
+            Time = transactionTime,
             Type = request.Type,
             StockCode = request.StockCode,
             Amount = response.TotalAmount,