KeepassXC is multiplatform. (Also: KeepassDX is quite nice as an an Android app for Keepass databases.)

There is a difference with adding an instance and accessing it that way (via their API - bypassing the server you signed up on) and via federation using the server you signed up on. Only in the latter case can you vote using the account already registered.

Try using the search feature to access the community via federation (e.g. “!cats@lemmy.world”) rather than adding the instance directly.