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/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;