[Bug Fix] Add Avatar image fallback handling

[djalmaaraujo]

Summary

• Add a CSP-friendly Stimulus controller for Avatar load/error handling
• Hide AvatarImage until it loads successfully and keep AvatarFallback visible on load failures
• Register the docs controller copy and add component markup coverage

Closes #259

Testing

• docker run --rm -v ruby_ui_bundle:/usr/local/bundle -v /Users/djalmaaraujo/dev/linkana/ruby_ui-issue-259:/workspace -w /workspace/gem ruby:3.4.7 bash -lc 'bundle install && bundle exec rake test TEST=test/ruby_ui/avatar_test.rb'