feat: 更新启动背景和主题样式，优化加载界面

lib/main.dart

@@ -11,6 +11,10 @@ import 'package:webview_flutter_wkwebview/webview_flutter_wkwebview.dart';
 import 'config/app_config.dart';
 
 final _homeUrl = AppConfig.homeUrl;
+const _shellBackground = Color(0xFFF8FBFF);
+const _shellAccent = Color(0xFF0089FF);
+const _shellSubText = Color(0xFF8E9AB0);
+const _resumeCoverDuration = Duration(milliseconds: 700);
 const _stopWebMediaScript = r'''
 (() => {
   try {
@@ -31,7 +35,7 @@ void main() {
     const SystemUiOverlayStyle(
       statusBarColor: Colors.transparent,
       statusBarIconBrightness: Brightness.dark,
-      systemNavigationBarColor: Colors.white,
+      systemNavigationBarColor: _shellBackground,
       systemNavigationBarIconBrightness: Brightness.dark,
     ),
   );
@@ -48,7 +52,7 @@ class ImWebViewApp extends StatelessWidget {
       debugShowCheckedModeBanner: false,
       theme: ThemeData(
         colorScheme: ColorScheme.fromSeed(seedColor: const Color(0xFF1F6FEB)),
-        scaffoldBackgroundColor: Colors.white,
+        scaffoldBackgroundColor: _shellBackground,
         useMaterial3: true,
       ),
       home: const H5ShellPage(),
@@ -68,6 +72,8 @@ class _H5ShellPageState extends State<H5ShellPage> with WidgetsBindingObserver {
 
   int _progress = 0;
   String? _loadError;
+  bool _showShellCover = true;
+  Timer? _shellCoverTimer;
 
   @override
   void initState() {
@@ -78,6 +84,7 @@ class _H5ShellPageState extends State<H5ShellPage> with WidgetsBindingObserver {
 
   @override
   void dispose() {
+    _shellCoverTimer?.cancel();
     WidgetsBinding.instance.removeObserver(this);
     super.dispose();
   }
@@ -89,6 +96,8 @@ class _H5ShellPageState extends State<H5ShellPage> with WidgetsBindingObserver {
         state == AppLifecycleState.paused ||
         state == AppLifecycleState.detached) {
       unawaited(_stopWebMedia());
+    } else if (state == AppLifecycleState.resumed) {
+      _showTransientShellCover();
     }
   }
 
@@ -113,7 +122,7 @@ class _H5ShellPageState extends State<H5ShellPage> with WidgetsBindingObserver {
       },
     )
       ..setJavaScriptMode(JavaScriptMode.unrestricted)
-      ..setBackgroundColor(Colors.white)
+      ..setBackgroundColor(_shellBackground)
       ..setNavigationDelegate(
         NavigationDelegate(
           onProgress: (progress) {
@@ -126,18 +135,25 @@ class _H5ShellPageState extends State<H5ShellPage> with WidgetsBindingObserver {
               setState(() {
                 _loadError = null;
                 _progress = 0;
+                _showShellCover = true;
               });
             }
           },
           onPageFinished: (_) {
             if (mounted) {
-              setState(() => _progress = 100);
+              setState(() {
+                _progress = 100;
+                _showShellCover = false;
+              });
             }
           },
           onWebResourceError: (error) {
             if (error.isForMainFrame ?? true) {
               if (mounted) {
-                setState(() => _loadError = error.description);
+                setState(() {
+                  _loadError = error.description;
+                  _showShellCover = false;
+                });
               }
             }
           },
@@ -170,6 +186,18 @@ class _H5ShellPageState extends State<H5ShellPage> with WidgetsBindingObserver {
     return controller;
   }
 
+  void _showTransientShellCover() {
+    _shellCoverTimer?.cancel();
+    if (mounted) {
+      setState(() => _showShellCover = true);
+    }
+    _shellCoverTimer = Timer(_resumeCoverDuration, () {
+      if (mounted) {
+        setState(() => _showShellCover = _progress < 100);
+      }
+    });
+  }
+
   Future<void> _runJavaScriptSafely(String source) async {
     try {
       await _controller.runJavaScript(source);
@@ -252,11 +280,17 @@ class _H5ShellPageState extends State<H5ShellPage> with WidgetsBindingObserver {
         }
       },
       child: Scaffold(
+        backgroundColor: _shellBackground,
         body: SafeArea(
           bottom: false,
           child: Stack(
             children: [
+              const Positioned.fill(child: _ShellFallback()),
               WebViewWidget(controller: _controller),
+              if (_showShellCover)
+                const Positioned.fill(
+                  child: IgnorePointer(child: _ShellFallback()),
+                ),
               if (_progress < 100)
                 LinearProgressIndicator(
                   value: _progress == 0 ? null : _progress / 100,
@@ -315,3 +349,133 @@ class _ErrorPanel extends StatelessWidget {
     );
   }
 }
+
+class _ShellFallback extends StatelessWidget {
+  const _ShellFallback();
+
+  @override
+  Widget build(BuildContext context) {
+    return DecoratedBox(
+      decoration: const BoxDecoration(
+        gradient: LinearGradient(
+          begin: Alignment.topCenter,
+          end: Alignment.bottomCenter,
+          colors: [
+            Color(0xFFE7F4FF),
+            _shellBackground,
+            Colors.white,
+          ],
+          stops: [0, 0.44, 1],
+        ),
+      ),
+      child: Stack(
+        children: [
+          const Positioned(
+            top: 44,
+            right: -72,
+            child: _SoftCircle(size: 190, opacity: 0.42),
+          ),
+          const Positioned(
+            top: 210,
+            left: -44,
+            child: _SoftCircle(size: 160, opacity: 0.34),
+          ),
+          const Positioned(
+            right: -54,
+            bottom: 250,
+            child: _SoftCircle(size: 220, opacity: 0.38),
+          ),
+          Center(
+            child: Column(
+              mainAxisSize: MainAxisSize.min,
+              children: const [
+                _ShellMark(),
+                SizedBox(height: 26),
+                Text(
+                  '心有回响',
+                  style: TextStyle(
+                    color: _shellAccent,
+                    fontSize: 26,
+                    height: 1.2,
+                    fontWeight: FontWeight.w700,
+                    letterSpacing: 0,
+                  ),
+                ),
+                SizedBox(height: 10),
+                Text(
+                  '正在为你唤醒会话',
+                  style: TextStyle(
+                    color: _shellSubText,
+                    fontSize: 13,
+                    height: 1.2,
+                    letterSpacing: 0,
+                  ),
+                ),
+                SizedBox(height: 32),
+                SizedBox(
+                  width: 110,
+                  child: LinearProgressIndicator(
+                    minHeight: 3,
+                    backgroundColor: Color(0xFFDCEEFF),
+                    color: _shellAccent,
+                  ),
+                ),
+              ],
+            ),
+          ),
+        ],
+      ),
+    );
+  }
+}
+
+class _ShellMark extends StatelessWidget {
+  const _ShellMark();
+
+  @override
+  Widget build(BuildContext context) {
+    return Container(
+      width: 126,
+      height: 126,
+      decoration: BoxDecoration(
+        shape: BoxShape.circle,
+        gradient: const LinearGradient(
+          begin: Alignment.topLeft,
+          end: Alignment.bottomRight,
+          colors: [Color(0xFFA8DDFF), _shellAccent, Color(0xFF0066E8)],
+        ),
+        boxShadow: [
+          BoxShadow(
+            color: _shellAccent.withValues(alpha: 0.2),
+            blurRadius: 34,
+            offset: const Offset(0, 18),
+          ),
+        ],
+      ),
+      child: const Icon(
+        Icons.check_rounded,
+        color: Colors.white,
+        size: 82,
+      ),
+    );
+  }
+}
+
+class _SoftCircle extends StatelessWidget {
+  const _SoftCircle({required this.size, required this.opacity});
+
+  final double size;
+  final double opacity;
+
+  @override
+  Widget build(BuildContext context) {
+    return Container(
+      width: size,
+      height: size,
+      decoration: BoxDecoration(
+        shape: BoxShape.circle,
+        color: const Color(0xFFBFE4FF).withValues(alpha: opacity),
+      ),
+    );
+  }
+}