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/composables/biddingItems.ts

@@ -0,0 +1,50 @@
+import { createSharedComposable } from "@vueuse/core";
+
+export type BiddingItem = {
+  id: number;
+  name: string;
+  startPrice: number;
+  remarks: string;
+  imageUrl?: string;
+};
+
+const _useBiddingItems = () => {
+  const biddingItems = ref<BiddingItem[]>([]);
+
+  const biddingItemsLatestId = computed(() => {
+    const arr = biddingItems.value ?? [];
+    if (arr.length === 0) return 1;
+    return Math.max(...arr.map((i) => i.id)) + 1;
+  });
+
+  const addBiddingItem = (item: Item, startPrice: number) => {
+    biddingItems.value.push({
+      id: biddingItemsLatestId.value,
+      name: `${item.brand} ${item.name}`,
+      startPrice: startPrice,
+      remarks: "",
+      imageUrl: item.imageUrl,
+    });
+  };
+
+  const removeBiddingItem = (id: number) => {
+    biddingItems.value = (biddingItems.value ?? []).filter((i) => i.id !== id);
+  };
+
+  const editBiddingItem = (biddingItem: BiddingItem) => {
+    const idx = (biddingItems.value ?? []).findIndex(
+      (i) => i.id === biddingItem.id
+    );
+    if (idx === -1) return;
+    const updatedBiddingItems = [...(biddingItems.value ?? [])];
+    updatedBiddingItems[idx] = {
+      ...updatedBiddingItems[idx],
+      ...biddingItem,
+    };
+    biddingItems.value = updatedBiddingItems;
+  };
+
+  return { biddingItems, addBiddingItem, removeBiddingItem, editBiddingItem };
+};
+
+export const useBiddingItems = createSharedComposable(_useBiddingItems);

app/composables/items.ts

@@ -4,7 +4,7 @@ export type Item = {
   id: number;
   brand?: string;
   name: string;
-  imageUrl?: string | null;
+  imageUrl?: string;
   description?: string | null;
   tags?: string[] | null;
   createdAt?: Date;