fix: image URLs incorrectly using API base URL instead of image base URL

Images are served from MinIO directly, not through /prod-api proxy.
Add imageBaseUrl to config and use it in fixImageUrl() and WXML templates.
This fixes images not loading on order list and detail pages.

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

coupon/utils/config.js

@@ -4,6 +4,8 @@ const config = {
   // baseUrl: 'http://192.168.1.100:8039',
   // 生产环境（通过 nginx 代理，HTTPS + 域名）
   baseUrl: 'https://ruilaizipj.com/prod-api',
+  // 图片/文件访问基地址（不走 /prod-api 代理，MinIO 直接访问）
+  imageBaseUrl: 'https://ruilaizipj.com',
 }
 
 module.exports = config