feat: 实现默认审批人和抄送人功能

- 后端新增 getDefaultApprovers 接口，从审批流程配置中获取默认审批人和抄送人
- 前端申请页面加载时自动获取并显示默认审批人和抄送人
- 添加默认审批流程配置：1个审批人(闫继光) + 2个抄送人(孟启、程丽敏)
- 移除后端 support 目录下的自定义 Db 和 Model 类，使用 Webman 官方实现
- 前端选择页面优化：优先从 sessionStorage 恢复，否则加载后端默认配置

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/config/route.php

@@ -34,6 +34,7 @@ Route::group('/api/v1/approval', function () {
 Route::group('/api/v1/business-trip', function () {
     Route::get('/list', [app\controller\BusinessTripController::class, 'list']);
     Route::get('/detail/{id}', [app\controller\BusinessTripController::class, 'detail']);
+    Route::get('/default-approvers', [app\controller\BusinessTripController::class, 'getDefaultApprovers']);
     Route::post('/create', [app\controller\BusinessTripController::class, 'create']);
     Route::post('/update/{id}', [app\controller\BusinessTripController::class, 'update']);
     Route::post('/submit/{id}', [app\controller\BusinessTripController::class, 'submit']);