fix(mobile-app): 开屏图片改为保持原比例不拉伸

使用 BoxFit.contain 替代 BoxFit.cover，
图片保持原比例显示，不足部分用黑边填充。

🤖 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

@@ -298,7 +298,7 @@ class _SplashPageState extends ConsumerState<SplashPage> {
     return SizedBox.expand(
       child: Image.asset(
         'assets/images/splash_static/splash_$_staticImageIndex.jpg',
-        fit: BoxFit.cover,
+        fit: BoxFit.contain, // 不拉伸，保持原比例，黑边填充
       ),
     );
   }