fix: 改进 Flutter Web 加载指示器隐藏逻辑

使用五种方式确保加载指示器被正确隐藏：
- Flutter 第一帧渲染完成事件
- Flutter 引擎初始化检查
- 10秒超时保护
- 页面加载后检查 Flutter 视图
- 定期轮询检查 Flutter 视图

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

flutter_monisuo/build/web/flutter_bootstrap.js

@@ -37,6 +37,6 @@ _flutter.buildConfig = {"engineRevision":"e4b8dca3f1b4ede4c30371002441c88c12187e
 
 _flutter.loader.load({
   serviceWorkerSettings: {
-    serviceWorkerVersion: "1409326861" /* Flutter's service worker is deprecated and will be removed in a future Flutter release. */
+    serviceWorkerVersion: "1994875870" /* Flutter's service worker is deprecated and will be removed in a future Flutter release. */
   }
 });