i18n: RTL smoke tests, Portuguese locale and a translation guide

- Add rtl_smoke_test covering inventory, detail (lot sheet) and settings
  under Directionality.rtl, with an Arabic label and vernacular name.
- Ship pt.i18n.json (Portuguese) as proof the locale pipeline scales beyond
  es/en; wire its language tile in Settings.
- docs/i18n.md documents the Weblate-ready flow for adding locales.

apps/app_seeds/lib/ui/settings_screen.dart

@@ -33,6 +33,11 @@ class SettingsScreen extends StatelessWidget {
             selected: current == 'en',
             onTap: () => LocaleSettings.setLocale(AppLocale.en),
           ),
+          _LanguageTile(
+            label: t.settings.langPt,
+            selected: current == 'pt',
+            onTap: () => LocaleSettings.setLocale(AppLocale.pt),
+          ),
           ListTile(
             leading: const Icon(Icons.smartphone_outlined),
             title: Text(t.settings.systemLanguage),