fix(mobile): update guide_5 to jpg and add AnimatedSize for smooth transition

- 将 guide_5.png 改为 guide_5.jpg
- 使用 AnimatedSize 包裹推荐码输入框，切换选项时平滑过渡，消除跳跃闪烁

🤖 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/guide_page.dart

@@ -62,7 +62,7 @@ class _GuidePageState extends ConsumerState<GuidePage> {
       subtitle: '你的资产 · 安全透明 · 不可被篡改',
     ),
     GuidePageData(
-      imagePath: 'assets/images/guide_5.png',
+      imagePath: 'assets/images/guide_5.jpg',
       title: '欢迎加入',
       subtitle: '创建账号前的最后一步 · 请选择是否有推荐人',
     ),
@@ -563,9 +563,12 @@ class _WelcomePageContentState extends ConsumerState<_WelcomePageContent> {
           ),
         ),
         SizedBox(height: 14.h),
-        // 推荐码输入框
+        // 推荐码输入框 - 使用 AnimatedSize 平滑过渡
-        if (_hasReferrer)
+        AnimatedSize(
-          Container(
+          duration: const Duration(milliseconds: 200),
+          curve: Curves.easeInOut,
+          child: _hasReferrer
+              ? Container(
                   padding: EdgeInsets.symmetric(vertical: 8.h, horizontal: 4.w),
                   decoration: BoxDecoration(
                     border: Border(
@@ -611,6 +614,8 @@ class _WelcomePageContentState extends ConsumerState<_WelcomePageContent> {
                       ),
                     ],
                   ),
+                )
+              : const SizedBox.shrink(),
         ),
         SizedBox(height: 24.h),
         // 没有推荐人选项