Improve handling of disabled accounts

[Anty0]

Describe the bug Currently, disabled accounts remain in the database, but the system doesn't properly check for them during login or sign-up. Because the UserAccount model enforces a unique constraint on username for non-deleted users, re-registering a previously disabled user causes a constraint violation that isn't handled gracefully.

Expected behavior Users should be directed to the organization owner or server administrator to have their disabled account restored.