feat(splash): 添加视频播放失败时的 fallback 动画

当设备硬件解码器无法播放视频时（如部分华为老机型），
显示 Logo 缩放 + 文字淡入的 Flutter 动画作为替代方案。

- 添加 fallback 动画控制器和动画序列
- Logo 从小到大弹性缩放 + 透明度渐显
- 文字延迟淡入显示
- 动画时长 3 秒，支持跳过按钮
- 保持与视频相同的用户体验流程

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/mobile-app/lib/features/auth/presentation/pages/splash_page.dart

@@ -16,7 +16,8 @@ class SplashPage extends ConsumerStatefulWidget {
   ConsumerState<SplashPage> createState() => _SplashPageState();
 }
 
-class _SplashPageState extends ConsumerState<SplashPage> {
+class _SplashPageState extends ConsumerState<SplashPage>
+    with SingleTickerProviderStateMixin {
   /// 视频播放控制器
   VideoPlayerController? _videoController;
 
@@ -29,6 +30,21 @@ class _SplashPageState extends ConsumerState<SplashPage> {
   /// 是否已经开始跳转（防止重复跳转）
   bool _isNavigating = false;
 
+  /// 视频加载是否失败（使用 fallback 动画）
+  bool _videoFailed = false;
+
+  /// Fallback 动画控制器
+  AnimationController? _fallbackAnimationController;
+
+  /// Logo 缩放动画
+  Animation<double>? _logoScaleAnimation;
+
+  /// Logo 透明度动画
+  Animation<double>? _logoOpacityAnimation;
+
+  /// 文字透明度动画
+  Animation<double>? _textOpacityAnimation;
+
   @override
   void initState() {
     super.initState();
@@ -39,6 +55,7 @@ class _SplashPageState extends ConsumerState<SplashPage> {
   void dispose() {
     _videoController?.removeListener(_onVideoStateChanged);
     _videoController?.dispose();
+    _fallbackAnimationController?.dispose();
     super.dispose();
   }
 
@@ -108,11 +125,99 @@ class _SplashPageState extends ConsumerState<SplashPage> {
         }
       }
 
-      // 如果视频加载失败，直接进行跳转
+      // 如果视频加载失败，启动 fallback 动画
-      _navigateToNextPage();
+      _startFallbackAnimation();
     }
   }
 
+  /// 启动 fallback 动画（视频加载失败时使用）
+  void _startFallbackAnimation() {
+    debugPrint('[SplashPage] 启动 fallback 动画');
+
+    setState(() {
+      _videoFailed = true;
+    });
+
+    // 创建动画控制器，总时长 3 秒
+    _fallbackAnimationController = AnimationController(
+      vsync: this,
+      duration: const Duration(milliseconds: 3000),
+    );
+
+    // Logo 缩放动画：0.5 -> 1.0 -> 1.05 -> 1.0（弹性效果）
+    _logoScaleAnimation = TweenSequence<double>([
+      TweenSequenceItem(
+        tween: Tween<double>(begin: 0.5, end: 1.05)
+            .chain(CurveTween(curve: Curves.easeOutBack)),
+        weight: 40,
+      ),
+      TweenSequenceItem(
+        tween: Tween<double>(begin: 1.05, end: 1.0)
+            .chain(CurveTween(curve: Curves.easeInOut)),
+        weight: 10,
+      ),
+      TweenSequenceItem(
+        tween: ConstantTween<double>(1.0),
+        weight: 50,
+      ),
+    ]).animate(_fallbackAnimationController!);
+
+    // Logo 透明度动画：0 -> 1
+    _logoOpacityAnimation = TweenSequence<double>([
+      TweenSequenceItem(
+        tween: Tween<double>(begin: 0.0, end: 1.0)
+            .chain(CurveTween(curve: Curves.easeOut)),
+        weight: 30,
+      ),
+      TweenSequenceItem(
+        tween: ConstantTween<double>(1.0),
+        weight: 70,
+      ),
+    ]).animate(_fallbackAnimationController!);
+
+    // 文字透明度动画：延迟出现
+    _textOpacityAnimation = TweenSequence<double>([
+      TweenSequenceItem(
+        tween: ConstantTween<double>(0.0),
+        weight: 30,
+      ),
+      TweenSequenceItem(
+        tween: Tween<double>(begin: 0.0, end: 1.0)
+            .chain(CurveTween(curve: Curves.easeOut)),
+        weight: 20,
+      ),
+      TweenSequenceItem(
+        tween: ConstantTween<double>(1.0),
+        weight: 50,
+      ),
+    ]).animate(_fallbackAnimationController!);
+
+    // 动画完成后跳转
+    _fallbackAnimationController!.addStatusListener((status) {
+      if (status == AnimationStatus.completed) {
+        _navigateToNextPage();
+      }
+    });
+
+    // 开始动画
+    _fallbackAnimationController!.forward();
+
+    // 1秒后显示跳过按钮
+    Future.delayed(const Duration(seconds: 1), () {
+      if (mounted) {
+        setState(() {
+          _showSkipButton = true;
+        });
+      }
+    });
+  }
+
+  /// 跳过 fallback 动画
+  void _skipFallbackAnimation() {
+    _fallbackAnimationController?.stop();
+    _navigateToNextPage();
+  }
+
   /// 视频状态变化监听
   void _onVideoStateChanged() {
     if (_videoController == null) return;
@@ -197,6 +302,9 @@ class _SplashPageState extends ConsumerState<SplashPage> {
                 ),
               ),
             )
+          else if (_videoFailed)
+            // 视频加载失败，显示 fallback 动画
+            _buildFallbackAnimationView()
           else
             // 视频加载中显示 Logo
             _buildLoadingView(),
@@ -264,10 +372,80 @@ class _SplashPageState extends ConsumerState<SplashPage> {
     );
   }
 
+  /// 构建 fallback 动画视图（视频加载失败时使用）
+  Widget _buildFallbackAnimationView() {
+    return AnimatedBuilder(
+      animation: _fallbackAnimationController!,
+      builder: (context, child) {
+        return Container(
+          width: double.infinity,
+          height: double.infinity,
+          decoration: const BoxDecoration(
+            gradient: LinearGradient(
+              begin: Alignment.topCenter,
+              end: Alignment.bottomCenter,
+              colors: [
+                Color(0xFFFFF5E6),
+                Color(0xFFFFE4B5),
+              ],
+            ),
+          ),
+          child: Column(
+            mainAxisAlignment: MainAxisAlignment.center,
+            children: [
+              // Logo 带缩放和透明度动画
+              Opacity(
+                opacity: _logoOpacityAnimation?.value ?? 1.0,
+                child: Transform.scale(
+                  scale: _logoScaleAnimation?.value ?? 1.0,
+                  child: Image.asset(
+                    'assets/images/logo/app_icon.png',
+                    width: 128,
+                    height: 128,
+                  ),
+                ),
+              ),
+              const SizedBox(height: 24),
+              // 文字带透明度动画
+              Opacity(
+                opacity: _textOpacityAnimation?.value ?? 1.0,
+                child: const Text(
+                  '榴莲皇后',
+                  style: TextStyle(
+                    fontSize: 32,
+                    fontFamily: 'Noto Sans SC',
+                    fontWeight: FontWeight.w700,
+                    height: 1.25,
+                    letterSpacing: 1.6,
+                    color: Color(0xFF4A3F35),
+                  ),
+                ),
+              ),
+              const SizedBox(height: 16),
+              // 副标题（延迟显示）
+              Opacity(
+                opacity: _textOpacityAnimation?.value ?? 1.0,
+                child: const Text(
+                  '认种榴莲 · 共享收益',
+                  style: TextStyle(
+                    fontSize: 16,
+                    fontFamily: 'Noto Sans SC',
+                    fontWeight: FontWeight.w400,
+                    color: Color(0xFF8B7355),
+                  ),
+                ),
+              ),
+            ],
+          ),
+        );
+      },
+    );
+  }
+
   /// 构建跳过按钮
   Widget _buildSkipButton() {
     return GestureDetector(
-      onTap: _skipVideo,
+      onTap: _videoFailed ? _skipFallbackAnimation : _skipVideo,
       child: Container(
         padding: const EdgeInsets.symmetric(horizontal: 16, vertical: 8),
         decoration: BoxDecoration(