feat: 支持x-login-as三参数认证、auth_url跟随base_url

- 认证请求增加x-login-as header支持连接账户
- auth_url根据base_url动态构建，不再硬编码生产环境
- 默认Base URL改为demo环境
- 设置页面新增Account ID字段，带tooltip说明

backend/app/services/airwallex_service.py

@@ -43,13 +43,14 @@ def get_client(db: Session) -> ProxiedAirwallexClient:
 
     client_id = _get_setting(db, "airwallex_client_id")
     api_key = _get_setting(db, "airwallex_api_key")
-    base_url = _get_setting(db, "airwallex_base_url", "https://api.airwallex.com/")
+    base_url = _get_setting(db, "airwallex_base_url", "https://api-demo.airwallex.com/")
+    login_as = _get_setting(db, "airwallex_login_as")
     proxy_url = _build_proxy_url(db)
 
     if not client_id or not api_key:
         raise HTTPException(status_code=400, detail="Airwallex 凭证未配置，请在系统设置中填写 Client ID 和 API Key")
 
-    config_hash = f"{client_id}:{api_key}:{base_url}:{proxy_url}"
+    config_hash = f"{client_id}:{api_key}:{base_url}:{login_as}:{proxy_url}"
     if _client_instance and _client_config_hash == config_hash:
         return _client_instance
 
@@ -60,11 +61,16 @@ def get_client(db: Session) -> ProxiedAirwallexClient:
         except Exception:
             pass
 
+    # auth_url must match base_url domain
+    auth_url = base_url.rstrip("/") + "/api/v1/authentication/login"
+
     _client_instance = ProxiedAirwallexClient(
         proxy_url=proxy_url,
+        login_as=login_as or None,
         client_id=client_id,
         api_key=api_key,
         base_url=base_url,
+        auth_url=auth_url,
     )
     _client_config_hash = config_hash
     return _client_instance