Fix: add-to-cart silent failure on expired nonce

dd_cart_add previously failed near-silently on an expired nonce on both desktop (product modal) and mobile (Menu page Screen 3) — the button reset with no message (desktop) or only logged to the console (mobile), and a retry click failed identically since the page's nonce doesn't refresh without a reload. Reused the dd_get_fresh_nonce endpoint added in v3.18.26 (no new server endpoint) — both call sites now retry exactly once with a freshly-fetched nonce on a success:false response, and show a real visible error if the retry or a genuine network error also fails. dd_cart_add's server-side nonce check is untouched — still protected, since this is a mutating action unlike the read-only endpoints fixed so far.