fix(ui): 初始背景跟随外观 + 修复跟随系统

- app.config 移除写死的浅色背景，交给 theme.json 按系统外观选择首帧底色
- 跟随系统模式增加 setInterval 轮询兜底，onThemeChange 不回调时也能跟上切换

dist/app.json

@@ -1 +1 @@
-{"darkmode":true,"themeLocation":"theme.json","pages":["pages/index/index","pages/shop/index","pages/shop/detail/index","pages/product/index","pages/diy/index","pages/diy/stickerEdit/index","pages/checkout/index","pages/designList/index","pages/orders/index","pages/profile/index","pages/service/index","pages/wordcloud/index","pages/orderDetail/index","pages/address/index","pages/settings/index","pages/agreement/index","pages/userDatabase/index"],"window":{"backgroundTextStyle":"dark","navigationStyle":"custom","navigationBarBackgroundColor":"#faf7f2","backgroundColor":"#faf7f2","backgroundColorTop":"#faf7f2","backgroundColorBottom":"#faf7f2","navigationBarTextStyle":"black"},"tabBar":{"custom":true,"list":[{"pagePath":"pages/index/index","text":"首页"},{"pagePath":"pages/shop/index","text":"商品"},{"pagePath":"pages/designList/index","text":"设计清单"},{"pagePath":"pages/orders/index","text":"订单"},{"pagePath":"pages/profile/index","text":"我的"}]}}
+{"darkmode":true,"themeLocation":"theme.json","pages":["pages/index/index","pages/shop/index","pages/shop/detail/index","pages/product/index","pages/diy/index","pages/diy/stickerEdit/index","pages/checkout/index","pages/designList/index","pages/orders/index","pages/profile/index","pages/service/index","pages/wordcloud/index","pages/orderDetail/index","pages/address/index","pages/settings/index","pages/agreement/index","pages/userDatabase/index"],"window":{"navigationStyle":"custom"},"tabBar":{"custom":true,"list":[{"pagePath":"pages/index/index","text":"首页"},{"pagePath":"pages/shop/index","text":"商品"},{"pagePath":"pages/designList/index","text":"设计清单"},{"pagePath":"pages/orders/index","text":"订单"},{"pagePath":"pages/profile/index","text":"我的"}]}}