Improve Errbit I18n #3130

Improve Errbit’s internationalization support for users and maintainers.

Goals:

• Discover selectable UI locales from config/locales/<locale>.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.