fix(pre-planting): 修复前后端 API 路径不匹配导致预种页面打不开

后端 PrePlantingController 缺少 eligibility 端点，前端请求 404 导致
Future.wait 整体失败，页面显示"加载数据失败"。

修复：
1. 后端: 在 PrePlantingController 添加 GET eligibility 端点
2. 前端: createOrder 路径从 /orders 改为 /purchase（匹配后端）
3. 前端: signMergeContract 路径从 /merges/:no/sign 改为 /sign-contract

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/services/planting-service/src/pre-planting/api/controllers/pre-planting.controller.ts

@@ -66,6 +66,13 @@ export class PrePlantingController {
     return { success: true };
   }
 
+  @Get('eligibility')
+  @ApiOperation({ summary: '查询当前用户预种购买资格' })
+  @ApiResponse({ status: HttpStatus.OK, description: '资格信息' })
+  async getEligibility(@Req() req: AuthenticatedRequest) {
+    return this.prePlantingService.getEligibility(req.user.accountSequence);
+  }
+
   @Get('position')
   @ApiOperation({ summary: '获取预种持仓信息' })
   @ApiResponse({ status: HttpStatus.OK, description: '持仓信息' })

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

@@ -10,14 +10,14 @@ import '../network/api_client.dart';
 //
 // === API 端点 ===
 // 所有端点走 planting-service 的 PrePlantingModule：
+// - GET  /pre-planting/config       获取预种开关状态（public controller）
+// - GET  /pre-planting/eligibility  检查购买资格
 // - GET  /pre-planting/position     获取预种持仓
-// - GET  /pre-planting/config       获取预种开关状态
-// - POST /pre-planting/orders       创建预种订单
-// - POST /pre-planting/orders/:no/pay  支付预种订单
 // - GET  /pre-planting/orders       获取预种订单列表
 // - GET  /pre-planting/merges       获取合并记录列表
-// - GET  /pre-planting/merges/:id   获取合并详情
+// - GET  /pre-planting/merges/:no   获取合并详情
-// - POST /pre-planting/merges/:id/sign  签署合并合同
+// - POST /pre-planting/purchase     创建预种订单（购买）
+// - POST /pre-planting/sign-contract  签署合并合同
 //
 // === 与现有 PlantingService 的关系 ===
 // 完全独立。PlantingService 处理整棵树认种（15831 USDT/棵），
@@ -386,7 +386,7 @@ class PrePlantingService {
       if (cityName != null) data['cityName'] = cityName;
 
       final response = await _apiClient.post(
-        '/pre-planting/orders',
+        '/pre-planting/purchase',
         data: data,
       );
 
@@ -496,7 +496,8 @@ class PrePlantingService {
     try {
       debugPrint('[PrePlantingService] 签署合并合同: mergeNo=$mergeNo');
       final response = await _apiClient.post(
-        '/pre-planting/merges/$mergeNo/sign',
+        '/pre-planting/sign-contract',
+        data: {'mergeNo': mergeNo},
       );
 
       if (response.statusCode == 200) {