I can sympathise as to why you feel so obligated to her, given her situation, but this has gone too far. She has an entitlement and dependency on you that is unlike healthy friendship.
What's important to understand OP, is that unpleasant or even abusive people can be vulernable or have been subject to past abuse themselves. This does not mean in any way that you'll have to also suffer being in their company. It is not your responsibility. She's an adult, not a child. You sound as if you're almost intimidated by her.
But it's just a quick question
This type of person is stressful for many people to deal with. They're constantly pushing boundaries and you fear the emotional punishment, rant or sulking that will follow your 'no'.
because an app that I pay for, and allow her to use my account for, wasn't working
Here's your option with the app -
a) Stop it immediately. Say you are no longer are subscribed to it because you don't find it of value anymore, and will not be subscribing just on her behalf no matter what she says. Do not get drawn into any offer of money to contribute.
b) Say that you're happy to carry on with her on the app, but you find her demands or requests for help stressful, and that if you have to deal with her having problems using it again, you'll regrettably have to deny her access for your own peace of mind.
She's not going to like either of these options, so take a deep breath and don't be drawn into arguments about it.
I don't even have to look at my phone when it rings these days as it's almost always her. Despite me telling her often how I can't deal with it and could she send a message instead. 'But it's just a quick question
Is it possible for you to put her on 'mute' instead of blocking her? That would allow you to check in on her message every few days. You'll probably have to tell her that you're doing this for your own emotional wellbeing though as she will just otherwise turn up to your house.
I'm afraid to be able to contain this, you'll have to risk losing her friendship, or upsetting her. She won't like it at all if you put your foot down and assert some communication boundaries.