bug: Redundant API calls for user deletion

## Refactor: Optimize User Deletion and Improve Error Handling

### Description
The current implementation of the `user delete` command is inefficient when deleting multiple users. It performs redundant API calls and lacks standardized error reporting via `RunE`.

### Current Issues

1. **Redundant API Calls:** For every username passed in the arguments, the command calls `GetUsersIdByName`. This function fetches the **entire list of users** every single time it is called.

```
for _, arg := range args {
// Retrieve user ID by name.
userID, err := api.GetUsersIdByName(arg)
if err != nil {
log.Errorf("failed to get user id for '%s': %v", arg, err)
continue
}
wg.Add(1)
go func(userID int64) {
defer wg.Done()
if err := api.DeleteUser(userID); err != nil {
errChan <- err
}
}(userID)
}
```

```
// Inside GetUsersIdByName
u, err := ListUsers(opts) // Fetches ALL users
for _, user := range u.Payload {
if user.Username == userName {
return user.UserID, nil
}
}
```

If a user deletes n accounts, the CLI fetches the full user list n times, making the subsequent concurrent deletion logic pointless due to the slow user id search phase.

2. **Inconsistent Output:**
The command does not use `RunE` or a standardized view to report success and failure messages, which is inconsistent with newer delete commands in the CLI.

### Proposed Changes

* **Batch Lookup:** Fetch the full list of users **only once** at the start of the command.
* **Standardize UI:** Use `RunE` and clear success/error messaging (similar to the logic used in newer delete command implementations).