Add customize-screen shop preview and purchase flow.

Locked items can be previewed on the pet from the editor, purchased with points in the preview modal, and opened in the full customize view from the shop.

lib/shop-catalog.ts

@@ -94,3 +94,10 @@ function getDefaultPriceForLayer(layerId: ShopLayerId, variantId: string) {
 export function getLayerLabel(layerId: ShopLayerId) {
   return CHARACTER_LAYERS.find((layer) => layer.id === layerId)?.label ?? layerId;
 }
+
+export function findShopItemById(
+  items: readonly ShopItemRecord[],
+  itemId: string,
+): ShopItemRecord | undefined {
+  return items.find((item) => item.id === itemId);
+}