feat(i18n): UI language selector (Settings → Appearance → Language) (T-462)

Make the localization usable: an app.locale select (English / Nederlands) in
the Appearance category, applied live by root_shell — it parses app.locale and
calls i18n.setLocale on boot + on settings change (setLocale is a no-op when
unchanged). nl_NL registered in availableLocales. The ui-design skill now
documents the locale-dir config and the rule to design for string-length
variation (translations run ~20% longer; never hard-size to the English label).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/widgets/src/typography.dart

@@ -44,6 +44,10 @@ const List<String> clideUiFamilyFallback = [
 const String kUiFontSettingKey = 'app.ui.font';
 const String kMonoFontSettingKey = 'app.mono.font';
 
+/// Settings key for the UI language (Settings → Appearance, T-462). Value is a
+/// `lang_country` string (e.g. `en_US`, `nl_NL`); unset → the default locale.
+const String kLocaleSettingKey = 'app.locale';
+
 // ---------------------------------------------------------------------------
 // Monospace face — JetBrains Mono
 // ---------------------------------------------------------------------------