fix(client): 联系/购买渠道卡点击外部打开(修空 onTap)+ 桌面账户子导航复现测试

- channel_launch: @→t.me / 邮箱→mailto / 域名→https,外部打开
- 联系页(桌面)/联系&兑换子页(移动)渠道卡接上 onTap
- account_desktop_nav_test: 证明桌面设置/兑换/联系/订单导航正常(231 全绿)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013nMthbVEmQquxBRKb9Fj8u

client/lib/screens/contact_page.dart

@@ -7,6 +7,7 @@ import 'package:flutter_riverpod/flutter_riverpod.dart';
 
 import '../l10n/app_text.dart';
 import '../pangolin_theme.dart';
+import '../services/channel_launch.dart';
 import '../state/app_providers.dart';
 import '../widgets/pangolin_icons.dart';
 
@@ -50,7 +51,9 @@ class _ChannelCard extends StatelessWidget {
   @override
   Widget build(BuildContext context) {
     final c = context.pangolin;
-    return Container(
+    return GestureDetector(
+      onTap: () => launchChannel(channel.sub),
+      child: Container(
       padding: const EdgeInsets.all(14),
       decoration: BoxDecoration(
         color: c.surface,
@@ -77,7 +80,7 @@ class _ChannelCard extends StatelessWidget {
         ),
         Icon(PangolinIcons.chevronRight, size: 18, color: c.fg3),
       ]),
-    );
+    ));
   }
 }