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.
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