feat: 商品详情页、XLS导入修复、分页选择器、导出功能

后端：
- 新增 product_images 表，支持每商品最多5张图（服务端压缩至1200px/JPEG85%）
- products 表新增 public_id(UUID)、description 字段
- 新增商品详情接口、二维码接口、公开商品接口（无鉴权）
- 修复 XLS 导入：OLE2 magic bytes 检测 + 临时文件解析，兼容 extrame/xls
- 修复商品/名称/系列/规格三张表导入数据为0（LastCol()=0 bug）
- 所有导入接口返回 total/imported/skipped 统计
- config 新增 StorageConfig，支持 STORAGE_* 环境变量覆盖
- 种子数据修复：products 补 public_id、新增 product_images TRUNCATE、schema.sql 表名修正

前端：
- 商品详情页：图片上传/删除、描述内联编辑、二维码弹窗、公开链接复制
- 公开商品页：无鉴权路由 /product/:public_id，Flutter Web SPA
- 商品详情列表（批次追踪）商品名超链接跳转详情页
- 导航「商品管理」改名「商品详情」
- 所有列表表格新增每页条数选择（10/20/50/100）
- 表格列头内嵌筛选（FilterableColumnHeader）
- 导出 Excel 功能（入库/出库/库存/财务/批次/往来单位）
- 网络恢复自动刷新 + 离线缓存展示

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

backend/internal/handler/product.go

@@ -1,12 +1,15 @@
 package handler
 
 import (
+	"bytes"
 	"errors"
 	"fmt"
 	"net/http"
 	"strconv"
 
 	"github.com/gin-gonic/gin"
+	"github.com/google/uuid"
+	qrcode "github.com/skip2/go-qrcode"
 	"gorm.io/gorm"
 
 	"github.com/wangjia/jiu/backend/internal/middleware"
@@ -65,6 +68,7 @@ func (h *ProductHandler) Create(c *gin.Context) {
 		return
 	}
 	product.ShopID = shopID
+	product.PublicID = uuid.New().String()
 
 	// Auto-generate product code if not provided (e.g. P001, P002)
 	// Retry up to 5 times on duplicate key to handle concurrent creates
@@ -130,6 +134,7 @@ func (h *ProductHandler) Update(c *gin.Context) {
 		"purchase_price": req.PurchasePrice,
 		"sale_price":     req.SalePrice,
 		"min_stock":      req.MinStock,
+		"description":    req.Description,
 		"remark":         req.Remark,
 		"custom_fields":  req.CustomFields,
 	}).Error; err != nil {
@@ -142,6 +147,97 @@ func (h *ProductHandler) Update(c *gin.Context) {
 	c.JSON(http.StatusOK, gin.H{"data": product})
 }
 
+// Detail GET /api/v1/products/:id/detail
+func (h *ProductHandler) Detail(c *gin.Context) {
+	shopID := middleware.GetShopID(c)
+	id := c.Param("id")
+
+	var product model.Product
+	if err := h.db.Where("id = ? AND shop_id = ? AND deleted_at IS NULL", id, shopID).
+		Preload("Category").Preload("Images").
+		First(&product).Error; err != nil {
+		c.JSON(http.StatusNotFound, gin.H{"error": "not found"})
+		return
+	}
+
+	// 老数据可能没有 public_id，按需补生成
+	if product.PublicID == "" {
+		product.PublicID = uuid.New().String()
+		h.db.Model(&product).Update("public_id", product.PublicID)
+	}
+
+	c.JSON(http.StatusOK, gin.H{"data": product})
+}
+
+// QRCode GET /api/v1/products/:id/qrcode
+func (h *ProductHandler) QRCode(c *gin.Context) {
+	shopID := middleware.GetShopID(c)
+	id := c.Param("id")
+
+	var product model.Product
+	if err := h.db.Where("id = ? AND shop_id = ? AND deleted_at IS NULL", id, shopID).
+		Select("id, public_id").First(&product).Error; err != nil {
+		c.JSON(http.StatusNotFound, gin.H{"error": "not found"})
+		return
+	}
+
+	url := "https://jiu.51yanmei.com/product/" + product.PublicID
+	png, err := qrcode.Encode(url, qrcode.Medium, 256)
+	if err != nil {
+		c.JSON(http.StatusInternalServerError, gin.H{"error": err.Error()})
+		return
+	}
+	c.DataFromReader(http.StatusOK, int64(len(png)), "image/png", bytes.NewReader(png), nil)
+}
+
+// FindOrCreate POST /api/v1/products/find-or-create
+func (h *ProductHandler) FindOrCreate(c *gin.Context) {
+	shopID := middleware.GetShopID(c)
+	var req struct {
+		Name   string `json:"name" binding:"required"`
+		Series string `json:"series"`
+		Spec   string `json:"spec"`
+	}
+	if err := c.ShouldBindJSON(&req); err != nil {
+		c.JSON(http.StatusBadRequest, gin.H{"error": err.Error()})
+		return
+	}
+
+	var product model.Product
+	err := h.db.Where("shop_id = ? AND name = ? AND series = ? AND spec = ? AND deleted_at IS NULL",
+		shopID, req.Name, req.Series, req.Spec).First(&product).Error
+	if err == nil {
+		c.JSON(http.StatusOK, gin.H{"data": product})
+		return
+	}
+	if !errors.Is(err, gorm.ErrRecordNotFound) {
+		c.JSON(http.StatusInternalServerError, gin.H{"error": err.Error()})
+		return
+	}
+
+	var count int64
+	h.db.Model(&model.Product{}).Where("shop_id = ? AND deleted_at IS NULL", shopID).Count(&count)
+	product = model.Product{
+		TenantBase: model.TenantBase{ShopID: shopID},
+		PublicID:   uuid.New().String(),
+		Name:       req.Name,
+		Series:     req.Series,
+		Spec:       req.Spec,
+		Code:       fmt.Sprintf("P%03d", count+1),
+	}
+	if createErr := h.db.Create(&product).Error; createErr != nil {
+		// Race condition: try to find the record created by another request
+		if h.db.Where("shop_id = ? AND name = ? AND series = ? AND spec = ? AND deleted_at IS NULL",
+			shopID, req.Name, req.Series, req.Spec).First(&product).Error == nil {
+			c.JSON(http.StatusOK, gin.H{"data": product})
+			return
+		}
+		c.JSON(http.StatusInternalServerError, gin.H{"error": createErr.Error()})
+		return
+	}
+	c.JSON(http.StatusCreated, gin.H{"data": product})
+}
+
 // Delete DELETE /api/v1/products/:id （软删除）
 func (h *ProductHandler) Delete(c *gin.Context) {
 	shopID := middleware.GetShopID(c)