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>

assets/i18n/en_us/builtin.theme-picker.json

@@ -18,5 +18,8 @@
   "settings.appearance.field.monoFont.label": { "translation": "Monospace font" },
   "settings.appearance.field.monoFont.help": { "translation": "Terminal, diffs, code, and IDs; applies live." },
   "settings.appearance.field.monoFont.option.jetBrainsMono": { "translation": "JetBrains Mono" },
-  "settings.appearance.field.monoFont.option.firaMono": { "translation": "Fira Mono" }
+  "settings.appearance.field.monoFont.option.firaMono": { "translation": "Fira Mono" },
+  "settings.appearance.section.language": { "translation": "Language" },
+  "settings.appearance.field.language.label": { "translation": "Language" },
+  "settings.appearance.field.language.help": { "translation": "Language for the app interface; applies live." }
 }