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>