fix: 优化供应商授权弹窗的标签和默认值

- 标签根据类型动态显示"分类"或"商品"
- 默认值改为undefined，显示placeholder而非0
- 切换类型时清空选择

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/views/admin/Suppliers.vue

@@ -44,7 +44,7 @@
             <el-option value="product" label="商品" />
           </el-select>
         </el-form-item>
-        <el-form-item label="关联项">
+        <el-form-item :label="authForm.type === 'category' ? '分类' : '商品'">
           <el-select v-model="authForm.ref_id" filterable placeholder="请选择" :loading="loadingOptions">
             <el-option
               v-for="item in optionsList"
@@ -80,7 +80,7 @@ const paginatedSuppliers = computed(() => {
   return suppliers.value.slice(start, start + pageSize)
 })
 const form = reactive({ username: '', email: '', password: '' })
-const authForm = reactive({ type: 'category', ref_id: 0 })
+const authForm = reactive<{ type: string; ref_id: number | undefined }>({ type: 'category', ref_id: undefined })
 const optionsList = ref<{ id: number; name: string }[]>([])
 const loadingOptions = ref(false)
 
@@ -103,13 +103,13 @@ async function addSupplier() {
 async function authorize(id: number) {
   authSupplierId.value = id
   authForm.type = 'category'
-  authForm.ref_id = 0
+  authForm.ref_id = undefined
   await loadOptions()
   showAuth.value = true
 }
 
 async function handleTypeChange() {
-  authForm.ref_id = 0
+  authForm.ref_id = undefined
   await loadOptions()
 }