fix(mobile-app): 修正认种价格为15831 USDT/棵

与后台 planting-service 保持一致

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

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

frontend/mobile-app/lib/core/services/planting_service.dart

@@ -100,7 +100,7 @@ class CreateOrderResponse {
       orderNo: json['orderNo'] ?? '',
       treeCount: json['treeCount'] ?? 0,
       totalAmount: (json['totalAmount'] ?? 0).toDouble(),
-      pricePerTree: (json['pricePerTree'] ?? 2199).toDouble(),
+      pricePerTree: (json['pricePerTree'] ?? 15831).toDouble(),
     );
   }
 }

frontend/mobile-app/lib/features/planting/presentation/pages/planting_quantity_page.dart

@@ -17,7 +17,7 @@ class PlantingQuantityPage extends ConsumerStatefulWidget {
 
 class _PlantingQuantityPageState extends ConsumerState<PlantingQuantityPage> {
   /// 每棵树的价格 (USDT)
-  static const double _pricePerTree = 2199.0;
+  static const double _pricePerTree = 15831.0;
 
   /// 可用余额 (USDT) - 从 API 获取
   double _availableBalance = 0.0;