Feature: short user and primary group names

### Is there an existing request for this feature?

- [X] I have searched the existing issues and found none that matched mine

### Describe the feature

Having the full email mapped to uid / gid results in a lot of wasted screen realestate.

```bash
Mick.Pollard@example.com@authd-01:~$ id
uid=67290(Mick.Pollard@example.com) gid=67290(Mick.Pollard@example.com)
```

```bash
drwxr-xr-x 2 Mick.Pollard@example.com Mick.Pollard@example.com 4.0K Aug 30 06:36 Desktop
drwxr-xr-x 2 Mick.Pollard@example.com Mick.Pollard@example.com 4.0K Aug 30 06:36 Documents
drwxr-xr-x 2 Mick.Pollard@example.com Mick.Pollard@example.com 4.0K Aug 30 06:36 Downloads
drwxr-xr-x 2 Mick.Pollard@example.com Mick.Pollard@example.com 4.0K Aug 30 06:36 Music
drwxr-xr-x 2 Mick.Pollard@example.com Mick.Pollard@example.com 4.0K Aug 30 06:36 Pictures
drwxr-xr-x 2 Mick.Pollard@example.com Mick.Pollard@example.com 4.0K Aug 30 06:36 Public
drwxr-xr-x 2 Mick.Pollard@example.com Mick.Pollard@example.com 4.0K Aug 30 06:36 Templates
drwxr-xr-x 2 Mick.Pollard@example.com Mick.Pollard@example.com 4.0K Aug 30 06:36 Videos

```

### Describe the ideal solution

I am not sure of how this could be implemented but a couple of thoughts:
* adding a way to allow to strip the domain part for the uid/gid number to name mapping OR
* adding an 'alias' field to the broker and use that when displaying username - `mick.pollard@brokeralias`

### Alternatives and current workarounds

I have no work around or alternative to offer at this stage.

### System information and logs

### Environment
* broker version: `0.1`
* authd version: `authd 0.3.3`
* gnome shell version: `46.3.1-1ubuntu1~24.04.1`
* Distribution: `Ubuntu`
* Distribution version: `24.04`

### Log files

n/a

#### Authd entries:

```raw
journalctl -u authd.service
```

#### MS Entra ID broker entries:

```raw
journalctl -u snap.authd-msentraid.authd-msentraid.service
```

### Application settings

#### Broker configuration:

```raw
# /var/snap/authd-msentraid/current/broker.conf
[oidc]
issuer = "https://login.microsoftonline.com/xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx/v2.0"
client_id = "xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx"

[users]
home_base_dir = "/home"
ssh_allowed_suffixes = "@example.com"
```

#### Broker authd configuration:

```raw
# /etc/authd/brokers.d/msentraid.conf
[authd]
name = Microsoft Entra ID
brand_icon = /snap/authd-msentraid/current/broker_icon.png
dbus_name = com.ubuntu.authd.MSEntraID
dbus_object = /com/ubuntu/authd/MSEntraID

```

### Relevant information

_No response_

### Double check your logs

- [X] I have redacted any sensitive information from the logs

Start by reproducing the long names with the reported `id` and directory listings, then review `/var/snap/authd-msentraid/current/broker.conf` and `/etc/authd/brokers.d/msentraid.conf` to understand the configured identity mapping. Done should mean a defined, consistent way to display shortened user and primary group names without changing the underlying identity.

Written by the indexing model from the issue text.