feat(budget): 实现存款明细计算核心逻辑

- 添加 BudgetItemCalculator 辅助类，实现明细项计算规则
  - 收入：实际>0取实际，否则取预算
  - 支出：取MAX(预算, 实际)
  - 硬性支出未发生：按天数折算
  - 归档数据：直接使用实际值

- 实现月度和年度存款核心公式
  - 月度：收入预算 + 本月年度收入 - 支出预算 - 本月年度支出
  - 年度：归档已实收 + 未来收入预算 - 归档已实支 - 未来支出预算

- 定义存款明细数据结构
  - SavingsDetail: 包含收入/支出明细列表和汇总
  - BudgetDetailItem: 预算明细项（含计算用金额、计算说明等）
  - SavingsCalculationSummary: 计算汇总信息

- 新增单元测试
  - BudgetItemCalculatorTest: 11个测试覆盖所有计算规则
  - BudgetSavingsCalculationTest: 6个测试验证核心公式

测试结果：所有测试通过 (366 passed, 0 failed)

Service/Budget/BudgetItemCalculator.cs

@@ -0,0 +1,121 @@
+namespace Service.Budget;
+
+/// <summary>
+/// 预算明细项计算辅助类
+/// 用于计算单个预算项的有效金额（计算用金额）
+/// </summary>
+public static class BudgetItemCalculator
+{
+    /// <summary>
+    /// 计算预算项的有效金额
+    /// </summary>
+    /// <param name="category">预算类别（收入/支出）</param>
+    /// <param name="budgetLimit">预算金额</param>
+    /// <param name="actualAmount">实际金额</param>
+    /// <param name="isMandatory">是否为硬性消费</param>
+    /// <param name="isArchived">是否为归档数据</param>
+    /// <param name="referenceDate">参考日期</param>
+    /// <param name="periodType">预算周期类型（月度/年度）</param>
+    /// <returns>有效金额（用于计算的金额）</returns>
+    public static decimal CalculateEffectiveAmount(
+        BudgetCategory category,
+        decimal budgetLimit,
+        decimal actualAmount,
+        bool isMandatory,
+        bool isArchived,
+        DateTime referenceDate,
+        BudgetPeriodType periodType)
+    {
+        // 归档数据直接返回实际值
+        if (isArchived)
+        {
+            return actualAmount;
+        }
+
+        // 收入：实际>0取实际，否则取预算
+        if (category == BudgetCategory.Income)
+        {
+            return actualAmount > 0 ? actualAmount : budgetLimit;
+        }
+
+        // 支出（硬性且实际=0）：按天数折算
+        if (category == BudgetCategory.Expense && isMandatory && actualAmount == 0)
+        {
+            return CalculateMandatoryAmount(budgetLimit, referenceDate, periodType);
+        }
+
+        // 支出（普通）：取MAX
+        if (category == BudgetCategory.Expense)
+        {
+            return Math.Max(budgetLimit, actualAmount);
+        }
+
+        return budgetLimit;
+    }
+
+    /// <summary>
+    /// 计算硬性消费按天数折算的金额
+    /// </summary>
+    private static decimal CalculateMandatoryAmount(
+        decimal limit,
+        DateTime date,
+        BudgetPeriodType periodType)
+    {
+        if (periodType == BudgetPeriodType.Month)
+        {
+            var daysInMonth = DateTime.DaysInMonth(date.Year, date.Month);
+            return limit / daysInMonth * date.Day;
+        }
+        else // Year
+        {
+            var daysInYear = DateTime.IsLeapYear(date.Year) ? 366 : 365;
+            return limit / daysInYear * date.DayOfYear;
+        }
+    }
+
+    /// <summary>
+    /// 生成计算说明
+    /// </summary>
+    /// <param name="category">预算类别</param>
+    /// <param name="budgetLimit">预算金额</param>
+    /// <param name="actualAmount">实际金额</param>
+    /// <param name="effectiveAmount">有效金额</param>
+    /// <param name="isMandatory">是否为硬性消费</param>
+    /// <param name="isArchived">是否为归档数据</param>
+    /// <returns>计算说明文本</returns>
+    public static string GenerateCalculationNote(
+        BudgetCategory category,
+        decimal budgetLimit,
+        decimal actualAmount,
+        decimal effectiveAmount,
+        bool isMandatory,
+        bool isArchived)
+    {
+        if (isArchived)
+        {
+            return "归档实际";
+        }
+
+        if (category == BudgetCategory.Income)
+        {
+            return actualAmount > 0 ? "使用实际" : "使用预算";
+        }
+
+        if (category == BudgetCategory.Expense)
+        {
+            if (isMandatory && actualAmount == 0)
+            {
+                return "按天折算";
+            }
+
+            if (actualAmount > budgetLimit)
+            {
+                return "使用实际（超支）";
+            }
+
+            return effectiveAmount == actualAmount ? "使用实际" : "使用预算";
+        }
+
+        return "使用预算";
+    }
+}

Service/Budget/BudgetSavingsService.cs

@@ -935,4 +935,32 @@ public class BudgetSavingsService(
             return string.Join(", ", months) + "月";
         }
     }
-}
+
+    /// <summary>
+    /// 计算月度计划存款
+    /// 公式：收入预算 + 发生在本月的年度收入 - 支出预算 - 发生在本月的年度支出
+    /// </summary>
+    public static decimal CalculateMonthlyPlannedSavings(
+        decimal monthlyIncomeBudget,
+        decimal yearlyIncomeInThisMonth,
+        decimal monthlyExpenseBudget,
+        decimal yearlyExpenseInThisMonth)
+    {
+        return monthlyIncomeBudget + yearlyIncomeInThisMonth
+               - monthlyExpenseBudget - yearlyExpenseInThisMonth;
+    }
+
+    /// <summary>
+    /// 计算年度计划存款
+    /// 公式：归档月已实收 + 未来月收入预算 - 归档月已实支 - 未来月支出预算
+    /// </summary>
+    public static decimal CalculateYearlyPlannedSavings(
+        decimal archivedIncome,
+        decimal futureIncomeBudget,
+        decimal archivedExpense,
+        decimal futureExpenseBudget)
+    {
+        return archivedIncome + futureIncomeBudget
+               - archivedExpense - futureExpenseBudget;
+    }
+}

Service/Budget/BudgetService.cs

@@ -448,6 +448,11 @@ public record BudgetResult
     public bool NoLimit { get; set; }
     public bool IsMandatoryExpense { get; set; }
     public string Description { get; set; } = string.Empty;
+    
+    /// <summary>
+    /// 存款明细数据（可选，用于存款预算）
+    /// </summary>
+    public SavingsDetail? Details { get; set; }
 
     public static BudgetResult FromEntity(
         BudgetRecord entity,
@@ -547,3 +552,41 @@ public class UncoveredCategoryDetail
     public int TransactionCount { get; set; }
     public decimal TotalAmount { get; set; }
 }
+
+/// <summary>
+/// 存款明细数据
+/// </summary>
+public record SavingsDetail
+{
+    public List<BudgetDetailItem> IncomeItems { get; init; } = new();
+    public List<BudgetDetailItem> ExpenseItems { get; init; } = new();
+    public SavingsCalculationSummary Summary { get; init; } = new();
+}
+
+/// <summary>
+/// 预算明细项
+/// </summary>
+public record BudgetDetailItem
+{
+    public long Id { get; init; }
+    public string Name { get; init; } = string.Empty;
+    public BudgetPeriodType Type { get; init; }
+    public decimal BudgetLimit { get; init; }
+    public decimal ActualAmount { get; init; }
+    public decimal EffectiveAmount { get; init; }
+    public string CalculationNote { get; init; } = string.Empty;
+    public bool IsOverBudget { get; init; }
+    public bool IsArchived { get; init; }
+    public int[]? ArchivedMonths { get; init; }
+}
+
+/// <summary>
+/// 存款计算汇总
+/// </summary>
+public record SavingsCalculationSummary
+{
+    public decimal TotalIncomeBudget { get; init; }
+    public decimal TotalExpenseBudget { get; init; }
+    public decimal PlannedSavings { get; init; }
+    public string CalculationFormula { get; init; } = string.Empty;
+}