feat: Link guest purchases to user accounts

Commit notes

This commit introduces functionality to automatically link guest purchases to user accounts upon registration or login. It also refactors the `hasPurchasedPack` function to efficiently handle both user ID and email lookups in a single query. Additionally, the frontend UI has been updated to better guide guest users towards account creation and to clearly communicate the benefits of doing so.

Co-authored-by: armin.naimi <[email redacted]>