Banned users should lose their profile

When a user is banned, we should get rid of their profile, and make sure querying their displayname + avatar return no values.

@pixlwave told me the apps are doing this app-side anyways, so we might as well put this logic inside the SDK instead.

No files or tests are named; start by locating the Rust SDK's ban handling and profile, display-name, and avatar query paths, then compare the app-side behavior mentioned in the issue. Done means a banned user's profile is removed and queries for that user's display name and avatar return no values.

Written by the indexing model from the issue text.