feat(i18n): add Brazilian Portuguese (pt-BR) locale (#810)

Adds a complete pt-BR translation (832 strings, full parity with en)
and registers it in the i18n config and language selector.

Co-authored-by: Claude Opus 4.8 (1M context) <[email protected]>

app/src/i18n/index.ts

@@ -3,11 +3,13 @@ import LanguageDetector from 'i18next-browser-languagedetector';
 import { initReactI18next } from 'react-i18next';
 import en from './locales/en/translation.json';
 import ja from './locales/ja/translation.json';
+import ptBR from './locales/pt-BR/translation.json';
 import zhCN from './locales/zh-CN/translation.json';
 import zhTW from './locales/zh-TW/translation.json';
 
 export const SUPPORTED_LANGUAGES = [
   { code: 'en', label: 'English' },
+  { code: 'pt-BR', label: 'Português (Brasil)' },
   { code: 'ja', label: '日本語' },
   { code: 'zh-CN', label: '简体中文' },
   { code: 'zh-TW', label: '繁體中文' },
@@ -21,6 +23,7 @@ i18n
   .init({
     resources: {
       en: { translation: en },
+      'pt-BR': { translation: ptBR },
       ja: { translation: ja },
       'zh-CN': { translation: zhCN },
       'zh-TW': { translation: zhTW },