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/translations.ts

@@ -319,6 +319,7 @@ export interface Translations {
     compare: string;
     share: string;
     notifyMe: string;
+    notifyMeConfirmed: string;
     zoom: string;
     fullscreen: string;
     pdfDocument: string;