feat: 更新 WebView 数据目录后缀至 v4；添加清除 WebView HTTP 缓存的方法

lib/main.dart

@@ -217,9 +217,18 @@ class _H5ShellPageState extends State<H5ShellPage> {
       });
     }
 
+    await _clearWebViewHttpCache();
     await _controller.loadRequest(Uri.parse(url));
   }
 
+  Future<void> _clearWebViewHttpCache() async {
+    try {
+      await _controller.clearCache();
+    } catch (_) {
+      // Some WebView implementations can reject cache operations before first use.
+    }
+  }
+
   Future<NavigationDecision> _handleNavigationRequest(
     NavigationRequest request,
   ) async {