feat:调整了启动/首屏阶段的 WebView 和壳层 fallback 展示：

WebView 背景从白色改成透明。
WebView 不再无条件渲染，只有已发起初始加载或首屏已展示后才渲染。
首屏未 ready 时，用 shell cover/fallback 盖住 WebView，避免白屏或 WebView 空白层提前露出。
顶部安全区背景色跟随 shell 背景，减少状态栏区域闪白

lib/main.dart

@@ -164,7 +164,7 @@ class _H5ShellPageState extends State<H5ShellPage> with WidgetsBindingObserver {
         onMessageReceived: (message) =>
             _handleFlutterShellMessage(generation, message),
       )
-      ..setBackgroundColor(Colors.white)
+      ..setBackgroundColor(Colors.transparent)
       ..setNavigationDelegate(
         NavigationDelegate(
           onProgress: (progress) {
@@ -1009,6 +1009,12 @@ class _H5ShellPageState extends State<H5ShellPage> with WidgetsBindingObserver {
     final shouldPaintShellFallback =
         (_isPreparingInitialLine || currentSlot.isAwaitingFirstScreen) &&
             currentSlot.loadError == null;
+    final shouldPaintShellCover =
+        currentSlot.showShellCover && !currentSlot.hasPresentedFirstScreen;
+    final shouldPaintWebView = currentSlot.hasLoadedInitialRequest ||
+        currentSlot.hasPresentedFirstScreen;
+    final shellBackgroundColor =
+        shouldPaintShellFallback ? _shellBackground : Colors.white;
 
     return PopScope(
       canPop: false,
@@ -1018,20 +1024,20 @@ class _H5ShellPageState extends State<H5ShellPage> with WidgetsBindingObserver {
         }
       },
       child: Scaffold(
-        backgroundColor: Colors.white,
+        backgroundColor: shellBackgroundColor,
         body: Stack(
           children: [
             SafeArea(
               bottom: false,
               child: Stack(
                 children: [
-                  if (shouldPaintShellFallback)
+                  if (shouldPaintShellFallback && !shouldPaintShellCover)
                     Positioned.fill(
                       child: _ShellFallback(progress: currentSlot.progress),
                     ),
+                  if (shouldPaintWebView)
                     Positioned.fill(child: _buildWebViewWidget()),
-                  if (currentSlot.showShellCover &&
-                      !currentSlot.hasPresentedFirstScreen)
+                  if (shouldPaintShellCover)
                     Positioned.fill(
                       child: IgnorePointer(
                         child: _ShellFallback(progress: currentSlot.progress),
@@ -1060,8 +1066,7 @@ class _H5ShellPageState extends State<H5ShellPage> with WidgetsBindingObserver {
               right: 0,
               height: topInset,
               child: ColoredBox(
-                color:
-                    shouldPaintShellFallback ? _shellBackground : Colors.white,
+                color: shellBackgroundColor,
               ),
             ),
           ],