fix: 更新商品代號驗證規則為 2-8 碼

1. ProductImport.php: 匯入規則調整
2. ProductController.php: 新增/編輯 API 規則調整
3. UI: 匯入與編輯視窗提示更新

app/Modules/Inventory/Imports/ProductImport.php

@@ -80,7 +80,7 @@ class ProductImport implements ToModel, WithHeadingRow, WithValidation, WithMapp
     public function rules(): array
     {
         return [
-            '商品代號' => ['required', 'string', 'min:1', 'max:5', 'unique:products,code'],
+            '商品代號' => ['required', 'string', 'min:2', 'max:8', 'unique:products,code'],
             '條碼' => ['required', 'string', 'unique:products,barcode'],
             '商品名稱' => ['required', 'string'],
             '類別名稱' => ['required', function($attribute, $value, $fail) {