fix: 修复库存导入 500 错误 + 开启全局文字选中

- findOrCreateProductFn 补充 PublicID(UUID)，避免 unique 约束冲突导致事务 500
- main.dart 加 SelectionArea builder，全局开启 Web 文字复制选中能力

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

backend/internal/handler/import.go

@@ -11,6 +11,7 @@ import (
 
 	"github.com/extrame/xls"
 	"github.com/gin-gonic/gin"
+	"github.com/google/uuid"
 	"github.com/xuri/excelize/v2"
 	"gorm.io/gorm"
 
@@ -703,6 +704,7 @@ func findOrCreateProductFn(db *gorm.DB, shopID uint64, name, series, spec string
 	}
 	p = model.Product{
 		TenantBase: model.TenantBase{ShopID: shopID},
+		PublicID:   uuid.New().String(),
 		Name:       name,
 		Series:     series,
 		Spec:       spec,

client/lib/main.dart

@@ -54,6 +54,8 @@ class _JiuAppState extends ConsumerState<JiuApp> {
       theme: AppTheme.light(),
       routerConfig: router,
       debugShowCheckedModeBanner: false,
+      builder: (context, child) =>
+          SelectionArea(child: child ?? const SizedBox()),
     );
   }
 }