feat(i18n): add Portuguese (Brazil) language support

[ygorsimoes]

Description

Add Portuguese (Brazil) localization to OpenCode using the br locale code, following the pattern from recent i18n PRs (#10018, #9887, #9882, #9947).

Changes

• Add br.ts translation files for app and UI packages (661 app keys, 93 UI keys)
• Update language context to support br locale
• Add br language label to all existing translations
• Language detection automatically maps pt, pt-br, pt-BR browser locales to br

Checklist

• All translations complete (661 app keys, 93 UI keys)
• Language detection works for pt, pt-br, pt-BR variants
• Code follows existing i18n patterns
• Ready for local testing and screenshots

Rationale for br locale code

Using br instead of pt-br for consistency with other single-word locale codes in the codebase and simplicity.

Would appreciate review and approval to proceed with the PR!