style(ui): 对齐所有页面样式到 Pencil 设计规范

- 移除硬编码颜色，使用主题变量
- market_page.dart: 统一卡片背景使用主题变量
- welfare_center_page.dart: 使用 AppColorScheme.warning
- 添加缺失的 import
- Light/Dark 模式完全适配

flutter_monisuo/build/web/flutter_bootstrap.js

@@ -37,6 +37,6 @@ _flutter.buildConfig = {"engineRevision":"e4b8dca3f1b4ede4c30371002441c88c12187e
 
 _flutter.loader.load({
   serviceWorkerSettings: {
-    serviceWorkerVersion: "3788064422" /* Flutter's service worker is deprecated and will be removed in a future Flutter release. */
+    serviceWorkerVersion: "759579798" /* Flutter's service worker is deprecated and will be removed in a future Flutter release. */
   }
 });