Create subscription — customer search now shows all customers

Two bugs in the manual subscription creation page (Subscriptions → Create subscription) have been fixed, based on a merchant report (SUB-7404):

The customer list was capped at 15 results. The customer search API used the platform's default page size and the frontend discarded the pagination data, with no way to load more. A store with 548 customers synced could see at most 15 of them — less than 3%. The picker now paginates using the same scroll-to-load pattern as the product search elsewhere in the app, so all customers are reachable.

The search field said "Search or create a customer." There is no customer-creation path in this flow: the modal opens a search-only interface, and no customer-create API call exists. The placeholder has been corrected to "Search for a customer."

Both fixes are now live for all merchants.