feat(export): implement export page for bidding items

Introduces a new export page for creating and managing bidding lists. Key features include: selecting items from a
master list, adding them to a bidding list, editing start price/remarks, batch price updates, and drag-and-drop
reordering. The final list can be previewed and exported as a CSV. This change adds the `useBiddingItems` composable and
the `sortablejs` dependency. Also refactors `imageUrl` to be a non-nullable string for type consistency.

app/components/export/EditModal.vue

@@ -0,0 +1,79 @@
+<template>
+  <UModal v-model:open="open" title="编辑标品">
+    <template #body>
+      <UForm
+        @submit="onSubmit"
+        :state="biddingItemEditState"
+        :schema="biddingItemEditSchema"
+      >
+        <UFormField label="起拍价" name="startPrice">
+          <UInput
+            v-model.number="biddingItemEditState.startPrice"
+            type="number"
+          />
+        </UFormField>
+
+        <UFormField label="备注" name="remarks">
+          <UTextarea v-model="biddingItemEditState.remarks" />
+        </UFormField>
+
+        <UButton type="submit" label="保存" color="primary" class="mt-4" />
+      </UForm>
+    </template>
+  </UModal>
+</template>
+
+<script lang="ts" setup>
+import type { FormSubmitEvent } from "@nuxt/ui";
+import * as z from "zod";
+
+const biddingItemEditForm = ref();
+const currentId = ref<number>(0);
+const open = ref<boolean>(false);
+
+const toast = useToast();
+
+const { biddingItems, editBiddingItem } = useBiddingItems();
+
+const biddingItemEditSchema = z.object({
+  startPrice: z.number().min(0, "起拍价不能少于 RM 0").default(0),
+  remarks: z.string().optional(),
+});
+
+type BiddingItemEditSchema = z.output<typeof biddingItemEditSchema>;
+
+const biddingItemEditState = reactive<BiddingItem & BiddingItemEditSchema>({
+  id: 0,
+  name: "",
+  imageUrl: "",
+  startPrice: 0,
+  remarks: "",
+});
+
+const openModal = (biddingItem: BiddingItem) => {
+  Object.assign(biddingItemEditState, biddingItem); // 👈 一次性复制所有字段
+  currentId.value = biddingItem.id;
+  open.value = true;
+};
+
+const onSubmit = async (event: FormSubmitEvent<BiddingItemEditSchema>) => {
+  const updatedItem: BiddingItem = {
+    ...biddingItemEditState, // 👈 这里直接复用已有字段
+    startPrice: event.data.startPrice,
+    remarks: event.data.remarks ?? "",
+  };
+
+  editBiddingItem(updatedItem);
+  toast.add({
+    title: "已保存修改",
+    description: `${updatedItem.name} 的信息已更新`,
+    color: "success",
+  });
+
+  open.value = false;
+};
+
+defineExpose({ openModal });
+</script>
+
+<style></style>