feat(config): 按业务模块拆分 Mock 开关

用 VITE_MOCK_* 系列环境变量替换全局 VITE_USE_MOCK，各模块独立控制演示数据
启用，并新增 src/utils/mock-flags.ts 统一解析缺省值。文档库关闭演示库时
如实提示接口待接入，避免静默沿用本地数据或覆盖已保存内容。

clinical-web/src/api/workbench/artifacts.ts

@@ -1,6 +1,7 @@
 import axios from 'axios'
 
 import { unwrapApiResponse } from '@/utils/api-response'
+import { mockFlags } from '@/utils/mock-flags'
 import { request } from '@/utils/request'
 
 import type {
@@ -11,7 +12,7 @@ import type {
   SigningArtifact,
 } from './types'
 
-const useMockData = import.meta.env.VITE_USE_MOCK !== 'false'
+const useMockData = mockFlags.signing
 
 type ArtifactCollection = SignArtifactResponseDto[] | ApiPageResponse<SignArtifactResponseDto>