feat(mobile-app): 启用兑换页面的划转功能

移除划转按钮的临时禁用标志，恢复正常功能。
用户点击划转按钮后将跳转到划转页面。

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/mobile-app/lib/features/trading/presentation/pages/trading_page.dart

@@ -562,29 +562,17 @@ class _TradingPageState extends ConsumerState<TradingPage> {
 
   /// 构建提取按钮
   /// 当 USDT 余额为 0 时禁用
-  /// [临时] 划转功能暂时禁用，待后续开放
   Widget _buildWithdrawButton() {
-    // [临时] 划转功能暂时禁用
-    const bool isTransferDisabled = true;
     // 余额为0时禁用
-    final bool canWithdraw = !_isLoading && _usdtBalance > 0 && !isTransferDisabled;
+    final bool canWithdraw = !_isLoading && _usdtBalance > 0;
     return Padding(
       padding: const EdgeInsets.symmetric(horizontal: 16),
       child: GestureDetector(
-        onTap: isTransferDisabled
+        onTap: canWithdraw
             ? () {
-                ScaffoldMessenger.of(context).showSnackBar(
-                  const SnackBar(
-                    content: Text('划转功能维护中，暂时无法使用'),
-                    backgroundColor: Color(0xFFD4AF37),
-                  ),
-                );
+                context.push(RoutePaths.withdrawUsdt);
               }
-            : canWithdraw
-                ? () {
-                    context.push(RoutePaths.withdrawUsdt);
-                  }
-                : null,
+            : null,
         child: Container(
           width: double.infinity,
           height: 56,