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.

package.json

@@ -12,6 +12,7 @@
   "dependencies": {
     "@nuxt/ui": "4.0.1",
     "nuxt": "^4.1.3",
+    "sortablejs": "^1.15.6",
     "typescript": "^5.9.3",
     "vue": "^3.5.22",
     "vue-router": "^4.5.1"