Palm Beach’s red-hot real estate market continues to sizzle on the high end — the very high end, in this case. The most expensive property ever listed on the island — a furnished seaside mansion priced at $140 million — has landed under contract. The just-completed contemporary-style residence on North County Road was listed for only a month. The property’s stratospheric asking price set an MLS record for a single-family property when it entered the market Jan. 8.