feat(P1-2): 预算三法并存落地 — 增量/零基/弹性均可应用

按任总观点: 三个方法都体现功能, 具体用哪个取决于用户场景
- 后端: POST /budget/apply-method 把所选方法结果写入预算计划(version标注方法)
  增量95.5万(上月×1.05) / 零基71.4万(成本砍半压缩) / 弹性58.7万(收入联动)
- 前端: 确认选择改为真正调用apply-method(原来只弹提示不落地)
- 场景提示: 增量=稳定快速编 / 零基=成本优化 / 弹性=收入波动大

frontend/src/api/index.ts

@@ -178,6 +178,7 @@ export const budgetApi = {
   autoDecompose: (data: any) => api.post('/budget/auto-decompose', data),
   deviationReport: (params?: any) => api.get('/budget/deviation-report', { params }),
   methodComparison: (data: any) => api.post('/budget/method-comparison', data),
+  applyMethod: (data: any) => api.post('/budget/apply-method', data),
   // 版本管理
   versions: (params?: any) => api.get('/budget/versions', { params }),
   versionSubmit: (data: any) => api.post('/budget/versions/submit', data),

frontend/src/views/BudgetManagement.vue

@@ -669,10 +669,23 @@ async function refreshMethodComparison() {
   } catch { ElMessage.error('加载预算方法对比失败') }
 }
 
-function confirmMethod() {
+async function confirmMethod() {
   const m = selectedMethodDetail.value
-  if (m) {
-    ElMessage.success(`已选择「${m.name}」，预算结果：${m.result_value} 万`)
+  if (!m) return
+  try {
+    const r: any = await budgetApi.applyMethod({
+      method: m.id,
+      year: currentYear,
+      entity: 'hanke',
+      entity_id: getEntityId(),
+      last_month_budget: 91,
+      current_revenue: 122,
+    })
+    ElMessage.success(r.message || `已应用「${m.name}」`)
+    // 刷新预算列表
+    loadBudget()
+  } catch (e: any) {
+    ElMessage.error(e?.response?.data?.detail || e?.message || '应用失败')
   }
 }