feat(mobile-app): 临时禁用划转功能

划转功能暂时维护中，点击按钮会显示提示信息。
恢复时将 isTransferDisabled 改为 false 即可。

🤖 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

@@ -562,17 +562,29 @@ 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;
+    final bool canWithdraw = !_isLoading && _usdtBalance > 0 && !isTransferDisabled;
     return Padding(
       padding: const EdgeInsets.symmetric(horizontal: 16),
       child: GestureDetector(
-        onTap: canWithdraw
+        onTap: isTransferDisabled
             ? () {
-                context.push(RoutePaths.withdrawUsdt);
+                ScaffoldMessenger.of(context).showSnackBar(
+                  const SnackBar(
+                    content: Text('划转功能维护中，暂时无法使用'),
+                    backgroundColor: Color(0xFFD4AF37),
+                  ),
+                );
               }
-            : null,
+            : canWithdraw
+                ? () {
+                    context.push(RoutePaths.withdrawUsdt);
+                  }
+                : null,
         child: Container(
           width: double.infinity,
           height: 56,