Username can be set to system user's name on instance with username-only mode enabled

Reproduction steps

Have a self-hosted instance with username-only mode enabled
Create an alt account with the same username to the system user (which should be the instance name)

Expected behaviour

The name should be restricted because it technically already exists as the system user has it

Actual behaviour

It allows you to use that username. I imagine this is because with username-only mode enabled, the system user still gets the #0000 discriminator instead of just the instance name

Build information

Stable Web 2026.1026.223737, Windows NT 10.0 (x64), Firefox 158.0, Locale en-US

Platform

Self-hosting