feat: add stickers delete api

lib/api.ts

@@ -459,6 +459,12 @@ export const rewardGifts = {
       { method: "POST", body: JSON.stringify({ content_type: contentType }) },
     ).then((r) => r.data),
 
+  /** Delete a custom sticker. Cascades out of every gallery and vault. */
+  remove: (key: string) =>
+    call<void>(`/admin/reward-gifts/${encodeURIComponent(key)}`, {
+      method: "DELETE",
+    }),
+
   /** Add a sticker to the catalog. It reaches nobody until it is sent. */
   create: (input: { name: string; meaning: string; image_url: string }) =>
     call<GiftableReward>("/admin/reward-gifts/catalog", {