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/widgets/account_screens.dart

@@ -10,6 +10,7 @@ import 'package:flutter_riverpod/flutter_riverpod.dart';
 import '../l10n/app_text.dart';
 import '../models/device.dart';
 import '../pangolin_theme.dart';
+import '../services/channel_launch.dart';
 import '../screens/settings_page.dart';
 import '../services/device_identity.dart';
 import 'adaptive_menu.dart';
@@ -429,7 +430,7 @@ class _RedeemScreenState extends ConsumerState<RedeemScreen> {
   Widget _channel(PangolinScheme c, IconData icon, String name, String sub, bool accent) {
     return InkWell(
       borderRadius: BorderRadius.circular(PangolinRadius.lg),
-      onTap: () {},
+      onTap: () => launchChannel(sub),
       child: Container(
         padding: const EdgeInsets.all(14),
         decoration: BoxDecoration(color: accent ? c.accentSubtle : c.surface, borderRadius: BorderRadius.circular(PangolinRadius.lg), border: Border.all(color: accent ? c.accentBorder : c.border), boxShadow: PangolinShadow.sm),
@@ -486,7 +487,7 @@ class ContactScreen extends StatelessWidget {
   Widget _contact(PangolinScheme c, IconData icon, String name, String sub, bool accent) {
     return InkWell(
       borderRadius: BorderRadius.circular(PangolinRadius.lg),
-      onTap: () {},
+      onTap: () => launchChannel(sub),
       child: Container(
         padding: const EdgeInsets.all(14),
         decoration: BoxDecoration(color: accent ? c.accentSubtle : c.surface, borderRadius: BorderRadius.circular(PangolinRadius.lg), border: Border.all(color: accent ? c.accentBorder : c.border), boxShadow: PangolinShadow.sm),