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),