Rename pt-BR locale to pt to match project convention

All existing locales use two-letter codes (ar, en, es, fr, nl) and the
crowdin.yml output path uses %two_letters_code% — using pt-BR would
conflict with any future Crowdin-imported Portuguese.

The display name stays 'Portuguese (Brazil)' / 'Português (Brasil)' so
users still see it's the Brazilian dialect.

src/contexts/LocalizationContext.jsx

@@ -25,7 +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)' },
+  { code: 'pt', name: 'Portuguese (Brazil)', nativeName: 'Português (Brasil)' },
 ]
 
 export const LocalizationProvider = ({ children }) => {