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/main.dart

@@ -394,6 +394,10 @@ Future<void> main() async {
     bundledThemes: themes,
     i18nLoader: AssetCatalogLoader(bundle: rootBundle),
     preloadNamespaces: _tier0Namespaces,
+    // Languages the UI can switch to (Settings → Appearance, T-462). Each needs
+    // an assets/i18n/<locale>/ catalog folder; root_shell applies the persisted
+    // app.locale on boot.
+    availableLocales: const [Locale('en', 'US'), Locale('nl', 'NL')],
     autoStartDaemonClient: false,
     toolchain: toolchain,
     minLogLevel: bootLogLevel,