feat: 添加启动屏幕图像和主题样式；优化 H5 壳的封面隐藏逻辑

lib/main.dart

@@ -99,6 +99,7 @@ class _H5ShellPageState extends State<H5ShellPage> {
   String? _loadError;
   bool _showShellCover = true;
   bool _shellBrandingLoaded = false;
+  Timer? _shellCoverFallbackTimer;
   late ShellBranding _shellBranding;
 
   @override
@@ -112,6 +113,10 @@ class _H5ShellPageState extends State<H5ShellPage> {
   WebViewController _buildController() {
     return WebViewController()
       ..setJavaScriptMode(JavaScriptMode.unrestricted)
+      ..addJavaScriptChannel(
+        'OpenIMShell',
+        onMessageReceived: _handleShellMessage,
+      )
       ..setBackgroundColor(_shellBackground)
       ..setNavigationDelegate(
         NavigationDelegate(
@@ -121,6 +126,7 @@ class _H5ShellPageState extends State<H5ShellPage> {
             }
           },
           onPageStarted: (_) {
+            _shellCoverFallbackTimer?.cancel();
             if (mounted) {
               setState(() {
                 _loadError = null;
@@ -134,6 +140,7 @@ class _H5ShellPageState extends State<H5ShellPage> {
           },
           onWebResourceError: (error) {
             if (error.isForMainFrame ?? true) {
+              _shellCoverFallbackTimer?.cancel();
               if (mounted) {
                 setState(() {
                   _loadError = error.description;
@@ -147,6 +154,12 @@ class _H5ShellPageState extends State<H5ShellPage> {
       );
   }
 
+  @override
+  void dispose() {
+    _shellCoverFallbackTimer?.cancel();
+    super.dispose();
+  }
+
   Future<void> _runJavaScriptSafely(String source) async {
     try {
       await _controller.runJavaScript(source);
@@ -165,9 +178,42 @@ class _H5ShellPageState extends State<H5ShellPage> {
     if (mounted) {
       setState(() {
         _progress = 100;
-        _showShellCover = false;
       });
     }
+    _scheduleShellCoverFallback();
+  }
+
+  void _handleShellMessage(JavaScriptMessage message) {
+    try {
+      final decoded = jsonDecode(message.message);
+      if (decoded is Map && decoded['type'] == 'first-screen-ready') {
+        _hideShellCover();
+      }
+    } catch (_) {
+      if (message.message == 'first-screen-ready') {
+        _hideShellCover();
+      }
+    }
+  }
+
+  void _scheduleShellCoverFallback() {
+    _shellCoverFallbackTimer?.cancel();
+    _shellCoverFallbackTimer = Timer(
+      const Duration(seconds: 4),
+      _hideShellCover,
+    );
+  }
+
+  void _hideShellCover() {
+    _shellCoverFallbackTimer?.cancel();
+    if (!mounted || !_showShellCover) {
+      return;
+    }
+
+    setState(() {
+      _progress = 100;
+      _showShellCover = false;
+    });
   }
 
   Future<void> _loadShellBrandingIfNeeded() async {
@@ -212,6 +258,7 @@ class _H5ShellPageState extends State<H5ShellPage> {
     final targetUrl = AppConfig.canonicalizeMainFrameUrl(url);
 
     if (mounted) {
+      _shellCoverFallbackTimer?.cancel();
       setState(() {
         _loadError = null;
         _progress = 0;