UI: Fix low value of std::clamp gets greater than high value

`std::clamp` was introduced at 60a45d3aa3 but it caused a runtime
assertion failure on Windows that checks the low value of `std::clamp`
is not greater than the high value.
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f