Integrate Weblate translation workflow #3131

Integrate Weblate to make Errbit translations easy to contribute and maintain.

Goals:

• Configure one Weblate component for Errbit-owned UI translations.
• Use config/locales/en.yml as the source and config/locales/*.yml as the mask.
• Do not create separate Weblate components for Rails or Devise translations.
• Import and preserve the existing Brazilian Portuguese translations.
• Keep standard Rails and Devise translations supplied by rails-i18n and devise-i18n.
• Exclude inactive, empty, untranslated, and fuzzy values from repository exports.
• Preserve translator comments, interpolation variables, and pluralization structure.
• Keep English source keys developer-owned.
• Synchronize translation changes through reviewed GitHub pull requests.
• Document the workflow in CONTRIBUTING.md and docs/i18n.md.
• Verify the complete source-to-Weblate-to-GitHub round trip.

This work depends on the catalog structure established by the I18n improvements issue.