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)

Web/src/components/Budget/BudgetChartAnalysis.vue

@@ -1013,6 +1013,7 @@ const yearBurndownChartOptions = computed(() => {
   display: flex;
   justify-content: center;
   align-items: center;
+  z-index: 1;
 }
 
 .gauge-text-overlay {
@@ -1048,6 +1049,8 @@ const yearBurndownChartOptions = computed(() => {
 
 .chart-header {
   margin-bottom: 12px;
+  position: relative;
+  z-index: 20;
 }
 
 .chart-title {

Web/src/views/budgetV2/Index.vue

@@ -123,6 +123,8 @@
             <SavingsBudgetContent
               v-else-if="activeTab === BudgetCategory.Savings"
               :budgets="savingsBudgets"
+              :income-budgets="incomeBudgets"
+              :expense-budgets="expenseBudgets"
               @savings-nav="handleSavingsNav"
             />
           </div>

Web/src/views/budgetV2/modules/SavingsBudgetContent.vue

@@ -183,6 +183,14 @@ const props = defineProps({
   budgets: {
     type: Array,
     default: () => []
+  },
+  incomeBudgets: {
+    type: Array,
+    default: () => []
+  },
+  expenseBudgets: {
+    type: Array,
+    default: () => []
   }
 })
 
@@ -199,6 +207,30 @@ const handleShowDetail = (budget) => {
   showDetailPopup.value = true
 }
 
+// 匹配收入预算
+const matchedIncomeBudget = computed(() => {
+  if (!currentBudget.value) {return null}
+  return props.incomeBudgets?.find(
+    b => b.periodStart === currentBudget.value.periodStart && b.type === currentBudget.value.type
+  )
+})
+
+// 匹配支出预算
+const matchedExpenseBudget = computed(() => {
+  if (!currentBudget.value) {return null}
+  return props.expenseBudgets?.find(
+    b => b.periodStart === currentBudget.value.periodStart && b.type === currentBudget.value.type
+  )
+})
+
+// 收入预算数据
+const incomeLimit = computed(() => matchedIncomeBudget.value?.limit || 0)
+const incomeCurrent = computed(() => matchedIncomeBudget.value?.current || 0)
+
+// 支出预算数据
+const expenseLimit = computed(() => matchedExpenseBudget.value?.limit || 0)
+const expenseCurrent = computed(() => matchedExpenseBudget.value?.current || 0)
+
 // 辅助函数
 const formatMoney = (val) => {
   return parseFloat(val || 0).toLocaleString(undefined, {