feat: real back-in-stock subscription for Notify Me (API + localStorage fallback)

notifyMe() just called toggleWishlist() - no actual subscription
mechanism existed. Now calls a new subscribeToRestock() API method
(POST /items/{id}/notify-me, not yet built server-side - see
BACKEND-API-REFERENCE.md §12.5) and falls back to a local-only record
in localStorage on failure, so the request isn't silently dropped
while the backend catches up.

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

src/app/i18n/en.ts

@@ -321,6 +321,7 @@ export const en: Translations = {
     compare: 'Compare',
     share: 'Share',
     notifyMe: 'Notify me',
+    notifyMeConfirmed: 'We\'ll let you know when this is back in stock.',
     zoom: 'Zoom',
     fullscreen: 'Fullscreen',
     pdfDocument: 'Product PDF document',