Granddaughter not showing any acknowledgment nor volunteering help

It must be strange for her too, living with her grandparents. Does she go home at weekends?
It depends what she was used to doing at home - if she wasn't expected to help then this could be a bone of contention.

Depends on why she is with you - it sounds as if she is taking out her dissatisfaction with life on you because you are nearest. Absolutely no reason why she shouldn’t do some chores and eat what she chooses from you providing what you normally eat. Be cheerful, accept no rudeness and don’t nag and you will all get there. Very tough being a teenager just now.

I had a rota on the kitchen board for mine.
Largely it was ignored (too much homework/got a music lesson/tennis/dancing/cricket etc) but it made me feel better.
The one thing they were supposed to do was tidy their rooms.
It was best not to inspect them though.

They're all very tidy now.

She is 16, not 5.

Based on that, I would agree with her that as you are roommates, you both hold responsibility on keeping up with the house, so make a chore calendar

No need to modify your eating habits.
She eats what she wants, keeps common areas clean and same principle applies for you.

Don't worry! It's a terrible age - hormones all over the place. I would not expect many youngsters of that age to show any interest in the household nor show any appreciation of anything. They are almost all like that. I have experience of children (now adults) and a grandchild who is a young teenager and can honestly say what you are experiencing is totally normal. There are some exceptions of course but mine were just as you describe. The good thing is that, give her a few years, your GD will probably have turned into a lovely, helpful young adult. Believe me it will happen. You have to grin and bear it for a while.

How annoying, bluerinse. However, it sounds as though she is only acting as a typical 16 year old.