Fix: User ID

[IeuanWalker]

Looks to be a possible bug with the MAUI preferences, before this PR we'd call the following code, to either get the saved user id or create/ save a new one -

Preferences.Get(userIdKey, GenerateNewAnonymousUserId());

But from testing, GenerateNewAnonymousUserId would always be called, even if one was already saved, which sets a new user ID -

public string GenerateNewAnonymousUserId()
{
    var userId = Guid.NewGuid().ToString();
    Preferences.Set(userIdKey, userId);

    return userId;
}

You can see what I mean in this screenshot -
(it first sets the user id to a custom value, it then gets the user ID, but it returns a new guide not the custom value)

To fix this I created a new method GetUserId() that gets the saved id, and then if its null, it will generate and save a new one.

Here is the updated version of the above screenshot -