Geez, at 5 she is still very impulsive. She was having a party and that was her focus. The rules weren't in her picture. This was something quite different then bringing a snack to her room.
I would've gone ahead with the party but I would also have told her this was an exception and told her that no food is an all the time rule.
Do you not ever make an exception for yourself? Say, you ordinarily don't eat in the living room but there's a special show on and so you eat dinner there this time.
I suggest canceling the party, while angry, was showing a lack of respect for her intent and her feelings. I'd apologize to her and arrange to have a party at some future time in the dining room. I'd tell her that I was upset because she'd not followed the rules but after I thought about it I realized she'd made a mistake and you understood her hurt feelings. You'd made a mistake too.
I do think suggesting to move the party was a good move and I would be irritated that she couldn't compromise but again, she's only 5 and hasn't yet learned much about compromise and making things work. A five year old can be very single minded just as you were with the rules.
I suggest both of you have hurt feelings and it would be helpful to "cry" together over the missed opportunity for a party.