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/item/EditModal.vue

@@ -100,7 +100,7 @@ const { utils, isNameAvailable, addItem, editItem, items } = useItemsStore();
 const baseSchema = z.object({
   brand: z.string().optional(),
   name: z.string().min(1, "名称不能为空"),
-  imageUrl: z.string().nullable().optional(),
+  imageUrl: z.string().optional(),
   description: z.string().nullable().optional(),
   tags: z.string().nullable().optional(),
 });
@@ -121,7 +121,7 @@ type ItemSchema = z.output<typeof baseSchema>;
 const itemState = reactive<ItemSchema>({
   brand: undefined,
   name: "",
-  imageUrl: null,
+  imageUrl: "",
   description: null,
   tags: null,
 });
@@ -134,7 +134,7 @@ const openModal = (item?: Item) => {
     currentId.value = item.id;
     itemState.brand = item.brand;
     itemState.name = item.name;
-    itemState.imageUrl = item.imageUrl ?? null;
+    itemState.imageUrl = item.imageUrl;
     itemState.description = item.description ?? null;
     itemState.tags = item.tags?.join(", ") ?? null;
   } else {
@@ -143,7 +143,7 @@ const openModal = (item?: Item) => {
     itemState.brand = undefined;
     currentId.value = null;
     itemState.name = "";
-    itemState.imageUrl = null;
+    itemState.imageUrl = "";
     itemState.description = null;
     itemState.tags = null;
   }