Add Brazilian Portuguese (pt-BR) translation

Adds public/locales/pt-BR/ with complete translations for common,
chores, and settings namespaces. Registers pt-BR in the
AVAILABLE_LANGUAGES picker in LocalizationContext.jsx.

Tone: informal 'você' form, natural Brazilian Portuguese.

src/contexts/LocalizationContext.jsx

@@ -25,6 +25,7 @@ export const AVAILABLE_LANGUAGES = [
   { code: 'es', name: 'Spanish', nativeName: 'Español' },
   { code: 'fr', name: 'French', nativeName: 'Français' },
   { code: 'nl', name: 'Dutch', nativeName: 'Nederlands' },
+  { code: 'pt-BR', name: 'Portuguese (Brazil)', nativeName: 'Português (Brasil)' },
 ]
 
 export const LocalizationProvider = ({ children }) => {