fix custom emoji selector often not completly visible

Summary

You're probably aware that the emoji dropdown is often partially outside the viewport. Closing it, scrolling a bit and then reopen it works most times, but on some small screens its not even possible.

I tried to fix it first with floating ui and popperjs, but could figure out a solution that works always. So instead now its now just always positioned in the middle of the screen, which seems to work in all cases now at least. I would suggest keeping it like that until someone figures it out with floating ui.

Screenshots: