I really feel for you!!!! It is so hard with the lockdown as well.
I am not sure what your finances can stretch to, but I would be planning some mini breaks for her, even if they are simple camping/gamping options. As she may meet other teens whilst away that she can hang out with, a change of scene and you can have fun together. I would do this as much as you possibly can.
Rope in family friends/family to come over at the weekends, or you can visit them. Even if they are not her age or have much in common, a changing sea of faces and interaction will be good for her.
Teen tennis camp and forest school for teens will still be running this year. It takes some confidence to go alone, but she may be up for it.
Print off some leaflets, and she can start a dog walking business over the summer, one way to meet others for sure.
Foodbanks need volunteers at the moment, she would be welcomed there with open arms. Give them a call.
Cookery courses aimed at young people are great for meeting other like minded teens.
Remember she has you, and you sound like a really great parent, so she is not alone by any stretch. It is one summer and may be one you remember because it can be very bonding. Remind her it is rubbish for everyone, and she is not alone with feeling this way. It is painful to watch I know op, but it is not forever 