Bid Beasts

Docs say auctions last 3 days, but code starts the timer only on the first bid and sets it to 15 minutes, with each new bid extending it by 15 minutes.
This causes issues:

• If no bid is placed, the auction never starts.

• If bidders keep bidding near the deadline, it could be extended indefinitely.

• The 3-day auction guarantee is broken.

Fix:
Set the auction end time when the NFT is listed:
listings[tokenId] = Listing({
seller: msg.sender,
minPrice: _minPrice,
buyNowPrice: _buyNowPrice,
auctionEnd: block.timestamp + 3 days,
listed: true
});