Improve Errbit I18n

Improve Errbit’s internationalization support for users and maintainers.

Goals:
- Discover selectable UI locales from `config/locales/.yml` catalogs.
- Keep Errbit-owned catalogs in `config/locales/*.yml`.
- Add `rails-i18n` and `devise-i18n` for standard framework translations.
- Remove redundant local Rails and standard Devise catalogs.
- Support per-user locale preferences with an Automatic option.
- Add browser `Accept-Language` detection, including anonymous Devise requests.
- Add English fallback behavior for missing translations.
- Preserve regional locales such as `pt-BR`.
- Localize dates, times, numbers, and pluralization.
- Replace remaining hardcoded Errbit UI strings with I18n keys.
- Keep API and notifier responses in English.
- Add catalog, request, profile, fallback, and regression tests.