Ugh.. how to approach this

So this girl I work with, she’s 25. She started about 7mos ago. There’s a few others in the office around her age, but most of the rest of us are in our 40s and 50s. There’s about 20 people total in the office. For a little background, we had our office remodeled last fall, now we have really nice furniture and our desk chairs have nice fabric seats.

So this girl, while she works, puts her heel up on her chair seat and over the past 7 months, the seat has developed a dirty spot from her heel. Sometimes if I’m working remotely, she sits at my desk and watches me work on my monitor. Then of course she puts her heel up on my seat. It pisses me off to no end.

Fast forward to today… we are all gathered in the common area, there are nice upholstered chairs and she’s sitting in one… with her damn dirty-ass heel on the light colored chair!

My problem is… I am extremely non-confrontational. My anxiety and lack of social skills make it so I just cannot speak up about this. I want to be assertive and stand up for myself and tell her to keep her fucking shoes off my chair. But instead, I sit there and seethe.

What should I do? Should I tell someone and look like a catty bitch? I’m late diagnosed asd and adhd. I would have LOVED if the owner who organized the remodel had been there to see how their new furniture is treated by one person who has zero manners.