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/i18n/strings_en.g.dart

@@ -211,6 +211,9 @@ class Translations$settings$en {
 	/// en: 'English'
 	String get langEn => 'English';
 
+	/// en: 'Português'
+	String get langPt => 'Português';
+
 	/// en: 'About'
 	String get about => 'About';
 
@@ -1508,6 +1511,7 @@ extension on Translations {
 			'settings.systemLanguage' => 'System language',
 			'settings.langEs' => 'Español',
 			'settings.langEn' => 'English',
+			'settings.langPt' => 'Português',
 			'settings.about' => 'About',
 			'settings.aboutText' => 'Local-first, encrypted inventory for traditional seeds. AGPL-3.0.',
 			'settings.aboutOpen' => 'About Tanemaki',