feat(ledger): 预种份额在流水明细中显示合并合同下载按钮

- 移除硬编码"预种无合同"逻辑
- PPL 份额点击详情时，查找是否有对应的已签署合并记录
- 有签署合同则显示查看/下载按钮，调用预种合并合同 PDF 接口
- 同时新增 _viewMergeContractPdf / _downloadMergeContractPdf 方法

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

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

@@ -8,6 +8,7 @@ import 'package:flutter_pdfview/flutter_pdfview.dart';
 import '../../../../core/di/injection_container.dart';
 import '../../../../core/services/wallet_service.dart';
 import '../../../../core/services/reward_service.dart';
+import '../../../../core/services/pre_planting_service.dart';
 import '../../../../core/utils/date_utils.dart';
 
 /// 账本明细页面 - 显示用户的流水账、统计图表和筛选功能
@@ -1116,18 +1117,40 @@ class _LedgerDetailPageState extends ConsumerState<LedgerDetailPage>
   Future<void> _showTransactionDetail(LedgerEntry entry) async {
     if (entry.refOrderId == null) return;
 
-    // 预种订单（PPL 前缀）没有合同，不显示合同按钮
     final bool isPrePlanting = entry.refOrderId!.startsWith('PPL');
 
+    // PPL 份额订单：查找是否已有对应的已签署合并合同
+    String? signedMergeNo;
+    if (isPrePlanting) {
+      try {
+        final merges = await ref.read(prePlantingServiceProvider).getMyMerges();
+        final matched = merges.where(
+          (m) => m.isSigned && m.sourceOrderNos.contains(entry.refOrderId),
+        );
+        if (matched.isNotEmpty) signedMergeNo = matched.first.mergeNo;
+      } catch (_) {
+        // 查不到就不显示合同按钮
+      }
+    }
+
+    if (!mounted) return;
     showModalBottomSheet(
       context: context,
       isScrollControlled: true,
       backgroundColor: Colors.transparent,
       builder: (context) => _TransactionDetailSheet(
         entry: entry,
-        showContractButtons: !isPrePlanting,
-        onViewContract: isPrePlanting ? null : () => _viewContractPdf(entry.refOrderId!),
-        onDownloadContract: isPrePlanting ? null : () => _downloadContractPdf(entry.refOrderId!),
+        showContractButtons: !isPrePlanting || signedMergeNo != null,
+        onViewContract: signedMergeNo != null
+            ? () => _viewMergeContractPdf(signedMergeNo!)
+            : isPrePlanting
+                ? null
+                : () => _viewContractPdf(entry.refOrderId!),
+        onDownloadContract: signedMergeNo != null
+            ? () => _downloadMergeContractPdf(signedMergeNo!)
+            : isPrePlanting
+                ? null
+                : () => _downloadContractPdf(entry.refOrderId!),
       ),
     );
   }
@@ -1230,6 +1253,89 @@ class _LedgerDetailPageState extends ConsumerState<LedgerDetailPage>
       );
     }
   }
+
+  /// 查看预种合并合同 PDF
+  Future<void> _viewMergeContractPdf(String mergeNo) async {
+    showDialog(
+      context: context,
+      barrierDismissible: false,
+      builder: (context) => const Center(
+        child: CircularProgressIndicator(
+          valueColor: AlwaysStoppedAnimation<Color>(Color(0xFFD4AF37)),
+        ),
+      ),
+    );
+
+    try {
+      final prePlantingService = ref.read(prePlantingServiceProvider);
+      final pdfBytes = await prePlantingService.downloadMergeContractPdf(mergeNo);
+
+      final tempDir = await getTemporaryDirectory();
+      final pdfFile = File('${tempDir.path}/merge_contract_$mergeNo.pdf');
+      await pdfFile.writeAsBytes(pdfBytes);
+
+      if (!mounted) return;
+      Navigator.of(context).pop();
+
+      Navigator.of(context).push(
+        MaterialPageRoute(
+          builder: (context) => _ContractPdfViewerPage(
+            pdfPath: pdfFile.path,
+            contractNo: mergeNo,
+          ),
+        ),
+      );
+    } catch (e) {
+      if (!mounted) return;
+      Navigator.of(context).pop();
+      ScaffoldMessenger.of(context).showSnackBar(
+        SnackBar(
+          content: Text('加载合同失败: $e'),
+          backgroundColor: Colors.red,
+        ),
+      );
+    }
+  }
+
+  /// 下载预种合并合同 PDF
+  Future<void> _downloadMergeContractPdf(String mergeNo) async {
+    showDialog(
+      context: context,
+      barrierDismissible: false,
+      builder: (context) => const Center(
+        child: CircularProgressIndicator(
+          valueColor: AlwaysStoppedAnimation<Color>(Color(0xFFD4AF37)),
+        ),
+      ),
+    );
+
+    try {
+      final prePlantingService = ref.read(prePlantingServiceProvider);
+      final pdfBytes = await prePlantingService.downloadMergeContractPdf(mergeNo);
+
+      final directory = await getApplicationDocumentsDirectory();
+      final fileName = 'merge_contract_$mergeNo.pdf';
+      final pdfFile = File('${directory.path}/$fileName');
+      await pdfFile.writeAsBytes(pdfBytes);
+
+      if (!mounted) return;
+      Navigator.of(context).pop();
+
+      await Share.shareXFiles(
+        [XFile(pdfFile.path)],
+        subject: '预种合并合同 - $mergeNo',
+      );
+    } catch (e) {
+      if (!mounted) return;
+      Navigator.of(context).pop();
+      ScaffoldMessenger.of(context).showSnackBar(
+        SnackBar(
+          content: Text('下载合同失败: $e'),
+          backgroundColor: Colors.red,
+        ),
+      );
+    }
+  }
 }
 
 /// 交易详情底部弹窗