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',
     ),
@@ -563,55 +563,60 @@ 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),
-            padding: EdgeInsets.symmetric(vertical: 8.h, horizontal: 4.w),
+          curve: Curves.easeInOut,
-            decoration: BoxDecoration(
+          child: _hasReferrer
-              border: Border(
+              ? Container(
-                bottom: BorderSide(
+                  padding: EdgeInsets.symmetric(vertical: 8.h, horizontal: 4.w),
-                  width: 1,
+                  decoration: BoxDecoration(
-                  color: Colors.white.withValues(alpha: 0.5),
+                    border: Border(
-                ),
+                      bottom: BorderSide(
-              ),
+                        width: 1,
-            ),
+                        color: Colors.white.withValues(alpha: 0.5),
-            child: Row(
-              children: [
-                Expanded(
-                  child: TextField(
-                    controller: _referralCodeController,
-                    decoration: InputDecoration(
-                      hintText: '请输入推荐码 / 序列号',
-                      hintStyle: TextStyle(
-                        fontSize: 16.sp,
-                        color: Colors.white.withValues(alpha: 0.6),
                       ),
-                      border: InputBorder.none,
-                      isDense: true,
-                      contentPadding: EdgeInsets.zero,
-                    ),
-                    style: TextStyle(
-                      fontSize: 16.sp,
-                      color: Colors.white,
-                    ),
-                    cursorColor: Colors.white,
-                  ),
-                ),
-                // 扫码按钮
-                GestureDetector(
-                  onTap: _openQrScanner,
-                  child: Padding(
-                    padding: EdgeInsets.only(left: 8.w),
-                    child: Icon(
-                      Icons.camera_alt_outlined,
-                      size: 20.sp,
-                      color: Colors.white.withValues(alpha: 0.8),
                     ),
                   ),
-                ),
+                  child: Row(
-              ],
+                    children: [
-            ),
+                      Expanded(
-          ),
+                        child: TextField(
+                          controller: _referralCodeController,
+                          decoration: InputDecoration(
+                            hintText: '请输入推荐码 / 序列号',
+                            hintStyle: TextStyle(
+                              fontSize: 16.sp,
+                              color: Colors.white.withValues(alpha: 0.6),
+                            ),
+                            border: InputBorder.none,
+                            isDense: true,
+                            contentPadding: EdgeInsets.zero,
+                          ),
+                          style: TextStyle(
+                            fontSize: 16.sp,
+                            color: Colors.white,
+                          ),
+                          cursorColor: Colors.white,
+                        ),
+                      ),
+                      // 扫码按钮
+                      GestureDetector(
+                        onTap: _openQrScanner,
+                        child: Padding(
+                          padding: EdgeInsets.only(left: 8.w),
+                          child: Icon(
+                            Icons.camera_alt_outlined,
+                            size: 20.sp,
+                            color: Colors.white.withValues(alpha: 0.8),
+                          ),
+                        ),
+                      ),
+                    ],
+                  ),
+                )
+              : const SizedBox.shrink(),
+        ),
         SizedBox(height: 24.h),
         // 没有推荐人选项
         GestureDetector(