Use uaccess to allow user access to the device

[abompard]

Ref: https://wiki.archlinux.org/title/Udev#Allowing_regular_users_to_use_devices

The uaccess udev tag will add an ACL on the device for the currently logged-in user. This means the rules must run before level 70, where the uaccess tag will be acted upon.

As a consequence, we don't need to create the uinput group, and the service can run in the per-user systemd instance.

Also, use an instanciated systemd service to handle multiple keyboards (I haven't tested that as I own only one).