Give a local chat message/alert to lurkers who wish to use a nickname that is already in use

There's been few occasions with random 'hi' users which does implicate they attempted to select a username that was already in use/registered. It would be beneficial to let those users know if the nickname is already registered/in use

as per latest: `I put in the nickname I wanted three times, and when I put in Hi suddenly it screws me`

Start by tracing the nickname selection and local chat or alert flow to find where an already-used nickname is handled. Confirm the behavior with a duplicate registered nickname and make sure the user receives a clear local message explaining that the nickname is unavailable.

Written by the indexing model from the issue text.