feat(budget): 添加存款明细数据生成逻辑

- 实现 GenerateMonthlyDetails 方法生成月度存款明细
  - 为每个预算项调用 BudgetItemCalculator 计算有效金额
  - 生成计算说明（使用预算/使用实际/超支/按天折算）
  - 标记超支项目
  - 生成汇总信息（总收入、总支出、计划存款）

- GetForMonthAsync 现在返回 Details 字段
  - 包含收入明细列表
  - 包含支出明细列表
  - 包含计算汇总和公式

- 新增集成测试验证 Details 字段生成正确
  - 验证收入项计算规则
  - 验证支出项超支标记
  - 验证硬性支出处理
  - 验证汇总计算

测试结果：58个预算测试全部通过

Service/Budget/BudgetSavingsService.cs

@@ -400,12 +400,25 @@ public class BudgetSavingsService(
             UpdateTime = dateTimeProvider.Now
         };
 
-        return BudgetResult.FromEntity(
+        // 生成明细数据
+        var referenceDate = new DateTime(year, month, dateTimeProvider.Now.Day);
+        var details = GenerateMonthlyDetails(
+            monthlyIncomeItems,
+            monthlyExpenseItems,
+            yearlyIncomeItems,
+            yearlyExpenseItems,
+            referenceDate
+        );
+
+        var result = BudgetResult.FromEntity(
             record,
             currentActual,
             new DateTime(year, month, 1),
             description.ToString()
         );
+        result.Details = details;
+
+        return result;
     }
 
     private async Task<BudgetResult> GetForYearAsync(
@@ -963,4 +976,144 @@ public class BudgetSavingsService(
         return archivedIncome + futureIncomeBudget
                - archivedExpense - futureExpenseBudget;
     }
+
+    /// <summary>
+    /// 生成月度存款明细数据
+    /// </summary>
+    private SavingsDetail GenerateMonthlyDetails(
+        List<(string name, decimal limit, decimal current, bool isMandatory)> monthlyIncomeItems,
+        List<(string name, decimal limit, decimal current, bool isMandatory)> monthlyExpenseItems,
+        List<(string name, decimal limit, decimal current)> yearlyIncomeItems,
+        List<(string name, decimal limit, decimal current)> yearlyExpenseItems,
+        DateTime referenceDate)
+    {
+        var incomeDetails = new List<BudgetDetailItem>();
+        var expenseDetails = new List<BudgetDetailItem>();
+
+        // 处理月度收入
+        foreach (var item in monthlyIncomeItems)
+        {
+            var effectiveAmount = BudgetItemCalculator.CalculateEffectiveAmount(
+                BudgetCategory.Income,
+                item.limit,
+                item.current,
+                item.isMandatory,
+                isArchived: false,
+                referenceDate,
+                BudgetPeriodType.Month
+            );
+
+            var note = BudgetItemCalculator.GenerateCalculationNote(
+                BudgetCategory.Income,
+                item.limit,
+                item.current,
+                effectiveAmount,
+                item.isMandatory,
+                isArchived: false
+            );
+
+            incomeDetails.Add(new BudgetDetailItem
+            {
+                Id = 0, // 临时ID
+                Name = item.name,
+                Type = BudgetPeriodType.Month,
+                BudgetLimit = item.limit,
+                ActualAmount = item.current,
+                EffectiveAmount = effectiveAmount,
+                CalculationNote = note,
+                IsOverBudget = item.current > 0 && item.current < item.limit,
+                IsArchived = false
+            });
+        }
+
+        // 处理月度支出
+        foreach (var item in monthlyExpenseItems)
+        {
+            var effectiveAmount = BudgetItemCalculator.CalculateEffectiveAmount(
+                BudgetCategory.Expense,
+                item.limit,
+                item.current,
+                item.isMandatory,
+                isArchived: false,
+                referenceDate,
+                BudgetPeriodType.Month
+            );
+
+            var note = BudgetItemCalculator.GenerateCalculationNote(
+                BudgetCategory.Expense,
+                item.limit,
+                item.current,
+                effectiveAmount,
+                item.isMandatory,
+                isArchived: false
+            );
+
+            expenseDetails.Add(new BudgetDetailItem
+            {
+                Id = 0,
+                Name = item.name,
+                Type = BudgetPeriodType.Month,
+                BudgetLimit = item.limit,
+                ActualAmount = item.current,
+                EffectiveAmount = effectiveAmount,
+                CalculationNote = note,
+                IsOverBudget = item.current > item.limit,
+                IsArchived = false
+            });
+        }
+
+        // 处理年度收入（发生在本月的）
+        foreach (var item in yearlyIncomeItems)
+        {
+            incomeDetails.Add(new BudgetDetailItem
+            {
+                Id = 0,
+                Name = item.name,
+                Type = BudgetPeriodType.Year,
+                BudgetLimit = item.limit,
+                ActualAmount = item.current,
+                EffectiveAmount = item.current, // 年度预算发生在本月的直接用实际值
+                CalculationNote = "使用实际",
+                IsOverBudget = false,
+                IsArchived = false
+            });
+        }
+
+        // 处理年度支出（发生在本月的）
+        foreach (var item in yearlyExpenseItems)
+        {
+            expenseDetails.Add(new BudgetDetailItem
+            {
+                Id = 0,
+                Name = item.name,
+                Type = BudgetPeriodType.Year,
+                BudgetLimit = item.limit,
+                ActualAmount = item.current,
+                EffectiveAmount = item.current,
+                CalculationNote = "使用实际",
+                IsOverBudget = item.current > item.limit,
+                IsArchived = false
+            });
+        }
+
+        // 计算汇总
+        var totalIncome = incomeDetails.Sum(i => i.EffectiveAmount);
+        var totalExpense = expenseDetails.Sum(e => e.EffectiveAmount);
+        var plannedSavings = totalIncome - totalExpense;
+
+        var formula = $"收入 {totalIncome:N0} - 支出 {totalExpense:N0} = {plannedSavings:N0}";
+
+        return new SavingsDetail
+        {
+            IncomeItems = incomeDetails,
+            ExpenseItems = expenseDetails,
+            Summary = new SavingsCalculationSummary
+            {
+                TotalIncomeBudget = totalIncome,
+                TotalExpenseBudget = totalExpense,
+                PlannedSavings = plannedSavings,
+                CalculationFormula = formula
+            }
+        };
+    }
 }