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/builtin/theme_picker/src/extension.dart

@@ -4,7 +4,7 @@ import 'package:clide/builtin/theme_picker/src/settings_view.dart';
 import 'package:clide/builtin/theme_picker/src/theme_status_item.dart';
 import 'package:clide/extension/extension.dart';
 import 'package:clide/kernel/kernel.dart';
-import 'package:clide/widgets/widgets.dart' show kMonoFontSettingKey, kUiFontSettingKey;
+import 'package:clide/widgets/widgets.dart' show kLocaleSettingKey, kMonoFontSettingKey, kUiFontSettingKey;
 
 class ThemePickerExtension extends ClideExtension {
   @override
@@ -101,6 +101,26 @@ class ThemePickerExtension extends ClideExtension {
               ),
             ],
           ),
+          SettingsSection(
+            label: 'Language',
+            labelKey: 'settings.appearance.section.language',
+            fields: [
+              SettingsField(
+                key: kLocaleSettingKey,
+                kind: SettingsFieldKind.select,
+                label: 'Language',
+                labelKey: 'settings.appearance.field.language.label',
+                help: 'Language for the app interface; applies live.',
+                helpKey: 'settings.appearance.field.language.help',
+                defaultValue: 'en_US',
+                // Language names stay in their own language (no labelKey).
+                options: [
+                  SettingsOption(value: 'en_US', label: 'English'),
+                  SettingsOption(value: 'nl_NL', label: 'Nederlands'),
+                ],
+              ),
+            ],
+          ),
         ],
       ),
     ),