fix(home-manager): assert qt.style.name for kvantum theme

[getchoo]

while having qt.enable be a prerequisite for this option applying
makes sense, the requirement on a specific style.name is not. this
make it more clear by moving that check to an assertion rather than an
internal comparison

[vdbe]

Is there a reason enablingqt.style.catppuccin does not set qt.style.name to kvantum itself.

If so I feel like this should be a warning.

[getchoo]

Is there a reason enabling qt.style.catppuccin does not set qt.style.name to kvantum itself.

i feel some options would be better set for the user - with this being one of them. setting qt.style.catppuccin doesn't really give the user any hint that kvantum will be enabled, which can easily lead to issues where they may set it to something, then have a conflict to deal with that contains no real guidance on why it was set to kvantum

If so I feel like this should be a warning.

i don't feel a warning is appropriate here as it's a hard requirement. if we can know for sure something is broken (like in the case of kvantum not being used), we should throw an evaluation error as to make it obvious qt.style.catppuccin will not work if enabled. after all, why would a user enable it if they were okay with it not doing anything?