fix(welcome): 修复欢迎页3个设计问题

1. Logo色与主色调不一致 — 品牌名"Genex"改用 ShaderMask 渐变
   (#9B5CF6→#D946EF)，与 logo icon 色系完全匹配
2. Logo到品牌名间距过大 — 24px → 12px，品牌名到slogan 8px → 6px
   符合 8pt grid 美学标准（紧凑型视觉组 4-8-12px）
3. 缺少微信登录 — 新增 WeChat 社交登录按钮（绿色#07C160），
   4语言i18n key: welcome.wechat

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

frontend/genex-mobile/lib/app/i18n/strings/en.dart

@@ -32,6 +32,7 @@ const Map<String, String> en = {
   'welcome.slogan': 'Make Every Coupon Count',
   'welcome.phoneRegister': 'Phone Sign Up',
   'welcome.emailRegister': 'Email Sign Up',
+  'welcome.wechat': 'WeChat',
   'welcome.otherLogin': 'Other Login Methods',
   'welcome.hasAccount': 'Already have an account?',
   'welcome.login': 'Log In',

frontend/genex-mobile/lib/app/i18n/strings/ja.dart

@@ -32,6 +32,7 @@ const Map<String, String> ja = {
   'welcome.slogan': 'すべてのクーポンに価値を',
   'welcome.phoneRegister': '電話番号で登録',
   'welcome.emailRegister': 'メールで登録',
+  'welcome.wechat': 'WeChat',
   'welcome.otherLogin': '他の方法でログイン',
   'welcome.hasAccount': 'アカウントをお持ちですか？',
   'welcome.login': 'ログイン',

frontend/genex-mobile/lib/app/i18n/strings/zh_cn.dart

@@ -32,6 +32,7 @@ const Map<String, String> zhCN = {
   'welcome.slogan': '让每一张券都有价值',
   'welcome.phoneRegister': '手机号注册',
   'welcome.emailRegister': '邮箱注册',
+  'welcome.wechat': '微信',
   'welcome.otherLogin': '其他方式登录',
   'welcome.hasAccount': '已有账号？',
   'welcome.login': '登录',

frontend/genex-mobile/lib/app/i18n/strings/zh_tw.dart

@@ -32,6 +32,7 @@ const Map<String, String> zhTW = {
   'welcome.slogan': '讓每一張券都有價值',
   'welcome.phoneRegister': '手機號註冊',
   'welcome.emailRegister': '信箱註冊',
+  'welcome.wechat': '微信',
   'welcome.otherLogin': '其他方式登入',
   'welcome.hasAccount': '已有帳號？',
   'welcome.login': '登入',

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

@@ -7,7 +7,7 @@ import '../../../../app/i18n/app_localizations.dart';
 
 /// A1. 欢迎页 - 品牌展示 + 注册/登录入口
 ///
-/// 品牌Logo、Slogan、手机号注册、邮箱注册、社交登录入口（Google/Apple）
+/// 品牌Logo、Slogan、手机号注册、邮箱注册、社交登录入口（WeChat/Google/Apple）
 class WelcomePage extends StatelessWidget {
   const WelcomePage({super.key});
 
@@ -30,17 +30,22 @@ class WelcomePage extends StatelessWidget {
                   height: 80,
                 ),
               ),
-              const SizedBox(height: 24),
+              const SizedBox(height: 12),
 
-              // Brand Name
-              Text(
-                'Genex',
-                style: AppTypography.displayLarge.copyWith(
-                  color: AppColors.primary,
-                  letterSpacing: 2,
+              // Brand Name — 使用 logo 同色系渐变
+              ShaderMask(
+                shaderCallback: (bounds) => const LinearGradient(
+                  colors: [Color(0xFF9B5CF6), Color(0xFFD946EF)],
+                ).createShader(bounds),
+                child: Text(
+                  'Genex',
+                  style: AppTypography.displayLarge.copyWith(
+                    color: Colors.white, // ShaderMask 需要白色底
+                    letterSpacing: 2,
+                  ),
                 ),
               ),
-              const SizedBox(height: 8),
+              const SizedBox(height: 6),
 
               // Slogan
               Text(
@@ -86,10 +91,19 @@ class WelcomePage extends StatelessWidget {
               ),
               const SizedBox(height: 16),
 
-              // Social Login Buttons
+              // Social Login Buttons — WeChat + Google + Apple
               Row(
                 mainAxisAlignment: MainAxisAlignment.center,
                 children: [
+                  _SocialLoginButton(
+                    icon: Icons.wechat,
+                    label: context.t('welcome.wechat'),
+                    color: const Color(0xFF07C160),
+                    onTap: () {
+                      Navigator.pushReplacementNamed(context, '/main');
+                    },
+                  ),
+                  const SizedBox(width: 24),
                   _SocialLoginButton(
                     icon: Icons.g_mobiledata_rounded,
                     label: 'Google',
@@ -146,12 +160,14 @@ class WelcomePage extends StatelessWidget {
 class _SocialLoginButton extends StatelessWidget {
   final IconData icon;
   final String label;
+  final Color? color;
   final VoidCallback onTap;
 
   const _SocialLoginButton({
     required this.icon,
     required this.label,
     required this.onTap,
+    this.color,
   });
 
   @override
@@ -164,11 +180,11 @@ class _SocialLoginButton extends StatelessWidget {
             width: 52,
             height: 52,
             decoration: BoxDecoration(
-              color: AppColors.gray50,
+              color: color != null ? color!.withValues(alpha: 0.1) : AppColors.gray50,
               shape: BoxShape.circle,
-              border: Border.all(color: AppColors.border),
+              border: Border.all(color: color?.withValues(alpha: 0.3) ?? AppColors.border),
             ),
-            child: Icon(icon, size: 28, color: AppColors.textPrimary),
+            child: Icon(icon, size: 28, color: color ?? AppColors.textPrimary),
           ),
           const SizedBox(height: 6),
           Text(label, style: AppTypography.caption),