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_image.go

@@ -0,0 +1,143 @@
+package handler
+
+import (
+	"fmt"
+	"image"
+	_ "image/jpeg"
+	_ "image/png"
+	"net/http"
+	"os"
+	"path/filepath"
+	"strconv"
+
+	"github.com/disintegration/imaging"
+	"github.com/gin-gonic/gin"
+	"github.com/google/uuid"
+	"gorm.io/gorm"
+
+	"github.com/wangjia/jiu/backend/config"
+	"github.com/wangjia/jiu/backend/internal/middleware"
+	"github.com/wangjia/jiu/backend/internal/model"
+)
+
+type ProductImageHandler struct {
+	db *gorm.DB
+}
+
+func NewProductImageHandler(db *gorm.DB) *ProductImageHandler {
+	return &ProductImageHandler{db: db}
+}
+
+// Upload POST /api/v1/products/:id/images
+func (h *ProductImageHandler) Upload(c *gin.Context) {
+	shopID := middleware.GetShopID(c)
+	productID, err := strconv.ParseUint(c.Param("id"), 10, 64)
+	if err != nil {
+		c.JSON(http.StatusBadRequest, gin.H{"error": "invalid product id"})
+		return
+	}
+
+	// Verify product belongs to shop
+	var product model.Product
+	if err := h.db.Where("id = ? AND shop_id = ? AND deleted_at IS NULL", productID, shopID).
+		First(&product).Error; err != nil {
+		c.JSON(http.StatusNotFound, gin.H{"error": "product not found"})
+		return
+	}
+
+	// Check image count limit
+	var count int64
+	h.db.Model(&model.ProductImage{}).Where("product_id = ? AND shop_id = ?", productID, shopID).Count(&count)
+	if count >= 5 {
+		c.JSON(http.StatusBadRequest, gin.H{"error": "最多上传 5 张图片"})
+		return
+	}
+
+	// Parse multipart (1MB limit)
+	if err := c.Request.ParseMultipartForm(1 << 20); err != nil {
+		c.JSON(http.StatusBadRequest, gin.H{"error": "文件超过 1MB 限制"})
+		return
+	}
+
+	file, _, err := c.Request.FormFile("file")
+	if err != nil {
+		c.JSON(http.StatusBadRequest, gin.H{"error": "请上传文件（field: file）"})
+		return
+	}
+	defer file.Close()
+
+	// Validate image format via decoding
+	img, _, err := image.Decode(file)
+	if err != nil {
+		c.JSON(http.StatusBadRequest, gin.H{"error": "仅支持 JPEG/PNG 图片"})
+		return
+	}
+
+	// Resize if needed (max 1200px on either dimension, preserve aspect ratio)
+	resized := imaging.Fit(img, 1200, 1200, imaging.Lanczos)
+
+	// Prepare output path
+	filename := uuid.New().String() + ".jpg"
+	subdir := fmt.Sprintf("%s/products/%d", config.C.Storage.UploadDir, productID)
+	if err := os.MkdirAll(subdir, 0755); err != nil {
+		c.JSON(http.StatusInternalServerError, gin.H{"error": "存储目录创建失败"})
+		return
+	}
+	fullPath := filepath.Join(subdir, filename)
+
+	// Save as JPEG with quality 85
+	if err := imaging.Save(resized, fullPath, imaging.JPEGQuality(85)); err != nil {
+		c.JSON(http.StatusInternalServerError, gin.H{"error": "图片保存失败"})
+		return
+	}
+
+	// Relative URL served by Nginx / dev static handler
+	relURL := fmt.Sprintf("/images/products/%d/%s", productID, filename)
+
+	pi := model.ProductImage{
+		ProductID: productID,
+		ShopID:    shopID,
+		URL:       relURL,
+	}
+	if err := h.db.Create(&pi).Error; err != nil {
+		os.Remove(fullPath)
+		c.JSON(http.StatusInternalServerError, gin.H{"error": err.Error()})
+		return
+	}
+
+	c.JSON(http.StatusCreated, gin.H{"data": pi})
+}
+
+// Delete DELETE /api/v1/products/:id/images/:image_id
+func (h *ProductImageHandler) Delete(c *gin.Context) {
+	shopID := middleware.GetShopID(c)
+	productID, err := strconv.ParseUint(c.Param("id"), 10, 64)
+	if err != nil {
+		c.JSON(http.StatusBadRequest, gin.H{"error": "invalid product id"})
+		return
+	}
+	imageID, err := strconv.ParseUint(c.Param("image_id"), 10, 64)
+	if err != nil {
+		c.JSON(http.StatusBadRequest, gin.H{"error": "invalid image id"})
+		return
+	}
+
+	var pi model.ProductImage
+	if err := h.db.Where("id = ? AND product_id = ? AND shop_id = ?", imageID, productID, shopID).
+		First(&pi).Error; err != nil {
+		c.JSON(http.StatusNotFound, gin.H{"error": "not found"})
+		return
+	}
+
+	// Delete file from disk
+	filename := filepath.Base(pi.URL)
+	fullPath := filepath.Join(config.C.Storage.UploadDir, fmt.Sprintf("products/%d/%s", productID, filename))
+	os.Remove(fullPath)
+
+	if err := h.db.Delete(&pi).Error; err != nil {
+		c.JSON(http.StatusInternalServerError, gin.H{"error": err.Error()})
+		return
+	}
+
+	c.JSON(http.StatusOK, gin.H{"message": "deleted"})
+}