feat(i18n): complete Portuguese, add Brazilian Portuguese (pt_BR)

Fill the ~74 UI strings pt.i18n.json was missing (saved searches, label
scanning, Plantaré, lot history), then add pt_BR as a proper Brazilian
variant rather than a copy of European Portuguese: tu/teu/tua -> você/
seu/sua with matching verb conjugation, partilhar -> compartilhar, and
Guardar -> Salvar for UI actions (seed-saving keeps "guardar", the
correct term in both variants). Wires AppLocale.ptBr into the language
picker and regenerates slang output.

apps/app_seeds/lib/ui/settings_screen.dart

@@ -57,6 +57,7 @@ List<(AppLocale, String)> _languages(Translations t) => [
   (AppLocale.ast, t.settings.langAst),
   (AppLocale.en, t.settings.langEn),
   (AppLocale.pt, t.settings.langPt),
+  (AppLocale.ptBr, t.settings.langPtBr),
   (AppLocale.fr, t.settings.langFr),
   (AppLocale.de, t.settings.langDe),
   (AppLocale.ja, t.settings.langJa),