fix(genex-mobile): Apple 登录按钮仅在 iOS 上显示

Apple Sign In 是 iOS/macOS 生态专属功能，Android 用户不存在
Apple ID 账号体系，不应在 Android 上展示该入口。

使用 defaultTargetPlatform == TargetPlatform.iOS 条件渲染：
- Android: WeChat + Google（2个按钮）
- iOS:     WeChat + Google + Apple（3个按钮）

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/genex-mobile/lib/features/auth/presentation/pages/welcome_page.dart

@@ -1,4 +1,5 @@
 import 'package:flutter/material.dart';
+import 'package:flutter/foundation.dart' show defaultTargetPlatform, TargetPlatform;
 import 'package:fluwx/fluwx.dart';
 import '../../../../app/theme/app_colors.dart';
 import '../../../../app/theme/app_typography.dart';
@@ -161,7 +162,9 @@ class _WelcomePageState extends State<WelcomePage> {
               ),
               const SizedBox(height: 16),
 
-              // Social Login Buttons — WeChat + Google + Apple
+              // Social Login Buttons
+              // Apple Sign In 仅在 iOS 上显示（Apple 账号体系限定 iOS/macOS 生态）
+              // Android 上只显示 WeChat + Google
               Row(
                 mainAxisAlignment: MainAxisAlignment.center,
                 children: [
@@ -180,14 +183,16 @@ class _WelcomePageState extends State<WelcomePage> {
                       Navigator.pushReplacementNamed(context, '/main');
                     },
                   ),
-                  const SizedBox(width: 24),
-                  _SocialLoginButton(
-                    icon: Icons.apple_rounded,
-                    label: 'Apple',
-                    onTap: () {
-                      Navigator.pushReplacementNamed(context, '/main');
-                    },
-                  ),
+                  if (defaultTargetPlatform == TargetPlatform.iOS) ...[
+                    const SizedBox(width: 24),
+                    _SocialLoginButton(
+                      icon: Icons.apple_rounded,
+                      label: 'Apple',
+                      onTap: () {
+                        Navigator.pushReplacementNamed(context, '/main');
+                      },
+                    ),
+                  ],
                 ],
               ),
               const SizedBox(height: 32),