fix(client): 公开页页脚「关于岩美」链接改为 AppConfig.aboutUrl 配置项

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

client/lib/core/config/app_config.dart

@@ -34,6 +34,11 @@ class AppConfig {
     defaultValue: '',
   );
 
+  static const aboutUrl = String.fromEnvironment(
+    'ABOUT_URL',
+    defaultValue: 'https://jiu.51yanmei.com',
+  );
+
   static String get baseUrl => _baseUrl;
   static String get apiBaseUrl => '$_baseUrl/api/v1';
   static String get healthUrl => '$_baseUrl/health';

client/lib/screens/public/public_product_screen.dart

@@ -1484,7 +1484,7 @@ class _Footer extends StatelessWidget {
               const _FooterDivider(),
               _FooterTextLink(text: '意见反馈', onTap: onFeedback),
               const _FooterDivider(),
-              _FooterTextLink(text: '关于岩美', url: 'https://jiu.51yanmei.com'),
+              _FooterTextLink(text: '关于岩美', url: AppConfig.aboutUrl),
             ],
           ),
         ],