v3.18.34 — dd_track_event nonce failures now observable

ajax_track_event() previously returned wp_send_json_success() unconditionally, even on nonce failure, making tracking failures invisible. Now returns a real 403/success:false on nonce failure, matching every other endpoint's response shape. No client-side change in this release — nothing currently reads the response — this only makes the failure observable for future retry logic. Also noted: cart.js's no-DDTrack fallback sends the wrong nonce type (dish_dash_frontend instead of dd_track) and was already failing before this fix; unchanged, tracked for a future pass. See RELEASE.md.