It doesn't matter what comes first if the user is the top priority. I've seen many projects fail because the needs of the user were not considered when building the Back-end first and once the Front-end was started, it was too late.
The projects I liked most had both parts started at the same time and every question was seen from a user perspective first.
In any case, the most important part was Front-end and Back-end developers first agreeing on a common goal and then a shared API between them.
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