I like the simplicity of your tool as well as the fact that no login is required to use it (although I would appreciate a backup option like saving the localStorage data as a single text file and being able to load it back).
A few suggestions about the UI:
Cards have no visual cue that they are stacked (I think this came up in some other replies here), so it is impossible to tell how much information can be found underneath. With books, for example, you can tell by their thickness roughly how many pages you can expect. But a general problem with digital content is that its dimensions can be infinite, especially if it is revealed/concealed interactively in a fractal-like manner as it is in your tool.
Personally, I would feel overwhelmed quickly after taking multiple nested notes, especially if I wanted to look up a piece of information that is hidden deep inside or if I wanted to reorganize my notes (although I think this is not possible yet in your app?).
I like that you made a breadcrumb-like navigation back to the parents - this helps a lot with orientation. But maybe also consider a more global navigation and a search tool to move more quickly between top-level notes and find notes that are deeply nested.
I would also adjust the font-size dependent on the amount of text in a card. If you enter a lot of text, cards quickly become too cumbersome to read.
I also think the parent card/header(?) should be differentiated much more against its child cards. I became a bit confused about which cards are clickable and which are not.
Nonetheless, I always appreciate people who build tools that they find useful for themself and share them with others, especially if they help with thinking.