fix: 区域统计客户筛选生效，后端region-stats支持过滤参数

- 后端/region-stats新增customer/city/region查询参数
- 前端regionFilters变化时重新请求后端数据
- 移除前端冗余过滤逻辑，由后端统一处理

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/api.ts

@@ -66,8 +66,13 @@ export async function fetchDeptStats(): Promise<DeptGroup[]> {
   return fetchJson<DeptGroup[]>(`${BASE}/dept-stats`);
 }
 
-export async function fetchRegionStats(): Promise<RegionGroup[]> {
-  return fetchJson<RegionGroup[]>(`${BASE}/region-stats`);
+export async function fetchRegionStats(params?: { customer?: string; city?: string; region?: string }): Promise<RegionGroup[]> {
+  const query = new URLSearchParams();
+  if (params?.customer) query.set('customer', params.customer);
+  if (params?.city) query.set('city', params.city);
+  if (params?.region) query.set('region', params.region);
+  const qs = query.toString();
+  return fetchJson<RegionGroup[]>(`${BASE}/region-stats${qs ? `?${qs}` : ''}`);
 }
 
 export async function fetchCustomerStats(): Promise<CustomerStats[]> {