feat(client): 商品详情加价格历史卡（接 /products/:id/price-history）

repo.getPriceHistory → 详情异步加载，新增价格历史卡：date + 进价，按相邻点
算涨跌（绿涨/红跌箭头）。至此商品详情 100% 对齐原型富数据版
（KPI + 各仓库分布 + 价格历史 + 近期流水）。golden 三主题已含该卡。

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TSKEiHsvauyxYUW2itzUXX

client/lib/repositories/product_repository.dart

@@ -87,6 +87,27 @@ class ProductRepository {
     }
   }
 
+  /// 进价历史点（date/price），按日期倒序。失败抛 AppException。
+  Future<List<({String date, double price})>> getPriceHistory(int id) async {
+    try {
+      final resp = await _client.get('/products/$id/price-history');
+      final data = (resp.data as Map<String, dynamic>)['data'] as List? ?? [];
+      return data.map((e) {
+        final m = e as Map<String, dynamic>;
+        final d = (m['date'] as String? ?? '');
+        return (
+          date: d.length >= 10 ? d.substring(0, 10) : d,
+          price: (m['price'] as num).toDouble(),
+        );
+      }).toList();
+    } on DioException catch (e) {
+      throw AppException(
+        e.response?.data?['error'] as String? ?? '获取价格历史失败',
+        statusCode: e.response?.statusCode,
+      );
+    }
+  }
+
   Future<ProductImage> uploadImage(int productId, String? filePath,
       {Uint8List? bytes, String? fileName}) async {
     try {