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/test/services/i18n_pt_br_locale_test.dart

@@ -0,0 +1,41 @@
+import 'package:flutter/widgets.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:tane/i18n/strings.g.dart';
+
+/// Brazilian Portuguese (`pt_BR`) is the first locale in the app to carry a
+/// region code alongside a base language that already has its own locale
+/// (`pt`, European Portuguese) — a case slang and the settings picker had
+/// never exercised before. These tests pin that both variants are wired
+/// distinctly and neither silently falls back to the other.
+void main() {
+  test('pt and pt_BR are both supported, as distinct locales', () {
+    expect(
+      AppLocaleUtils.supportedLocales,
+      containsAll([const Locale('pt'), const Locale('pt', 'BR')]),
+    );
+  });
+
+  test('core strings resolve in Brazilian Portuguese', () {
+    final ptBr = AppLocale.ptBr.buildSync();
+    expect(ptBr.menu.inventory, 'Inventário');
+    expect(ptBr.common.save, 'Salvar');
+    expect(ptBr.settings.langPtBr, 'Português (Brasil)');
+  });
+
+  test('pt_BR wording diverges from pt where Brazilian usage differs', () {
+    final pt = AppLocale.pt.buildSync();
+    final ptBr = AppLocale.ptBr.buildSync();
+    // European Portuguese uses "Guardar"; Brazilian software uses "Salvar".
+    expect(pt.common.save, 'Guardar');
+    expect(ptBr.common.save, 'Salvar');
+  });
+
+  test('pt_BR translates a key added after the initial pt rollout', () {
+    final en = AppLocale.en.buildSync();
+    final ptBr = AppLocale.ptBr.buildSync();
+    // savedSearches was a later addition; if pt_BR were missing it, this
+    // would silently surface the English fallback instead.
+    expect(ptBr.savedSearches.title, isNot(en.savedSearches.title));
+    expect(ptBr.savedSearches.title, isNotEmpty);
+  });
+}