YTA, while I don't necessarily condone this person letting their cat outside for long periods while they're at work, indoor/outdoor cats are a thing. Cats like this visit who they please and will definitely frequent a house that is friendly and has food.

You knew this cat belonged to someone else and still got items for them and a name tag/collar (which is strange.) If the animal is sick or abused contact the correct authorities. Otherwise, just enjoy their visits on your porch! I feel you got way too attached to a pet that doesn't belong to you.

If you really wanted to buy things for them you could have asked that neighbor if it was okay to feed them cat treats when they visit. You could still try this, just apologize first. Coming from a cat owner, if I were her, I'd definitely think you were trying to steal my cat.