Fix avatar image placement in Firefox

[lsouoliveira]

Fix the avatar image placement on Firefox. Issue #259.

Before:

After:

[izaguirrejoe]

Ping

[ghost]

As we talked in private, the problem is that AvatarFallback name is misleading, in the current code it isn't a fallback, it's just a element behind/above another element. I think we should implement a proper fallback behavior here using native HTML events (eg. onerror) and Stimulus controllers.

This solution doesn't fix the problem entirely, eg.:

Avatar do
  AvatarImage(src: "wrong-url", alt: "joeldrapper")
  AvatarFallback { "JD" }
end

This keeps breaking even with this PR.

[lsouoliveira]

I'm closing this PR for now. As Stephann suggested, a good next step would be to handle image loading errors and display the fallback dynamically. Someone else can pick this up and continue from here.