fix(languages): show error instead of silently no-oping when adding a duplicate locale

addLocale() always cleared the input, even when LocaleSyncService rejected
the code because it was already supported - same silent-failure shape as
the widgets JSON bug fixed earlier this session. Now checks locales()
first and shows an inline error, leaving the input untouched, instead of
clearing it like the add succeeded. Verified live: typing an existing
locale code and clicking Add now shows 'This language is already supported.'

Navigation-section was also audited (id generation, label locale-migration,
reorder swap, grouped-footer read-only fallback) - no defects found, it's
solid as-is.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/app/i18n/en.ts

@@ -679,6 +679,7 @@ export const en: Translations = {
     catalogNavTopCarousel: 'Top category carousel',
     catalogNavTopCarouselDesc: 'Horizontally scrollable category chips above the catalog grid.',
     newLanguageDesc: 'ISO language code to add as a supported storefront language (e.g. de, fr).',
+    languageAlreadyAdded: 'This language is already supported.',
     languagesTabDesc: 'Languages available to shoppers. The default language is used as a fallback everywhere.',
     linkLabelDesc: 'Text shown for this navigation link.',
     linkUrlDesc: 'Path or URL this navigation link points to.',