chore(ui): improve hash parsing + test - WF-72

It's possible to use `URLSearchParams` instead of complex Regex. As it's
a very testable part of the code, it was a good opportunity to introduce
Vitest and cover it.

This improvement made me realise that a bug was present, and we didn't
serialize the params when converting back the object to hash (`.key =
length` was not called).