Maybe just say - “I love you, can I make your bed for you, like mum and I used to do when you were little?”. That shows love and also makes the messy room look better. Promise not to rummage through her things - just put clean bedding on.
Try to keep giving her non judgemental support, have a boundary that there is no old food left in her filthy bedroom otherwise you will get flies or ants or worse still, mice or rats!
She sound so low that she needs non judgemental acts of loving parenting first, and even with this she might hate herself so much that she finds this unpleasant, and then after that you can provide gentle practical support (and bin her used pads for her). This is preferable to shouting at her for being dirty or lazy and will get a better turn around. I cannot say enough though, that a turn around will take time.
It is a tricky one.
It is a combination of being eagle eyed and watching out, but not looking like you are, about giving love like you are a saint and expecting nothing in return, and then combining this with having some boundaries so she knows where she stands.
I personally would choose to say that one meal a day is eaten together, and everyone clears away at least their own plate. I would also choose that shared rooms (like the living room) are not made messy, and that no doors are never slammed. I say everyone has to step outside every day to breathe fresh air, and they have to look up into the distance to stretch their focal length beyond their screens.
It is August, and she could be thinking about work or education for September. Maybe things will pick up naturally after things change in September.
Does your daughter have a mother figure in her life? It must be so hard for you to see this girl wasting days in a sad state. She is young yet and things will get better.
If pants are an issue, buy her an extra 5 pack of ordinary pants from the supermarket and say - they might not be the nicest ones but I thought of you, these might be useful. It is far less confrontational and can form part of an apology and recognition that what you have said in the past might have sounded judgemental when actually she just needs a little bit of help, like we all do.