[Bug] Adding a username with invalid charcters gives silence error

[feikedonia]

Dear Cosmic-team,

So I read #832 and thought how to brake it! (It's testing: I want you to have the best possible product!)

version
cosmic-session-git 1.0.0.alpha.4.r2.g0a3b9f5-1
linux 6.12.6.arch1-1

steps to reproduce

1. Settings -> System & Accounts -> Users -> Add user-button.
2. Fill in the Full name-field with a real name: "Real Name" for example.
3. Fill in the Username-field with some invalid characters, like: "\na$!#@" for example.
4. Fill in the Password-field with: "test"
5. Keep the Administrator-slide at unset
6. Press Add user
7. Validate the Authentication Required-popup
8. No user is added.

the problem

• a user may think he/she added an account, but it didn't .
• a user may not know which characters are invalid.

proposal
At step 6, when the user presses Add user, have the Username-field with the invalid characters, marked in red with a tool tip: "username may only contain lowercase letters, underscores and numbers"

Thank you

add user