I'm a front-end developer working as a member of an agile team. I have no clue how much of UI/UX must be done before every sprint and how much of it I can postpone for later.
There is no universal answer to this. However, we can draw some conclusions.
Some amount of design (both "front end" UI or UX design as well as "back end") must be done in order to refine a Product Backlog Item. Without some design information, it will be hard to add "detail, estimates, and order" to the Product Backlog Item while performing Product Backlog Refinement.
Some level of design, which may be more or not, must be done before Sprint Planning in order to enable a team to adequately determine what work can be done within the Sprint and to create a plan for completing that work.
Aside from that, it's up to the team. It's also a balancing act - more up-front design is time and energy away from producing working software, but it also increases knowledge and reduces risk. Some teams have a working agreement called the Definition of Ready (not defined in the Scrum Guide) that defines exactly what is the minimum bar for a Product Backlog Item to be ready for refinement and/or ready for Sprint Planning.