Folder ("category") tree for better managing subfolders

[SamuXzX]

Is your feature request related to a problem? Please describe.
Some time ago I reported issue #771 to let the user choose the root directory to store notes, so that the folder structure of the notes could be the same of the one used for the files.
Thank you very much for solving the issue.

Now we 2 problems that the come with such a solution:

1. Having nested folders is useful but the actual UI in Notes isn't sophisticated enough to support them: in the top-left corner we only see the first level of folders and all the nested becomes blocks in the notes list. To me this is not very comfortable for navigating between notes:
   

2. Given the fact that I have lots of .md files across my Nextcloud instance, Notes reasonably tries to load them all when I open a first-level category. This causes a slow-down in the loading of the page, which is okay, but sometimes makes Firefox alert me about the tab.
   

Describe the solution you'd like
It may be a good solution to implement a folder-tree. The Android App does it at least for the second level of nesting, then it divides the list in blocks like the web app does:

This could be an initial solution given that the list of categories remains the same when selecting a category. It would help with problem (1) making the navigation easier, and with problem (2) moving further the time the app needs to list all the notes in the subfolders as blocks in the notes list.

This seems like a partial solution. We could go even further listing the subfolders under the folders as subfolder/subsubfolder..., so that we don't need lots of level of indentation. Such lists of subfolders could be opened only when clicking on their relative parent, so that the folder tree doesn't takes space when not needed, and ellipsing the lesser level of nesting in case the name is too long so that the deeper level of nesting is recognizable:

[newhinton]

I made a similar suggestion, see #413 (comment)

[phil-lipp]

Sadly the Three column layout didn't resolve this. I also would highly appreciate a better handling of subfolders and I think with the extra space used for the new layout this could be implemented really well.

As for my use-case: the first level in the /Notes/ folder is for different categories (e.g. personal notes, vacation notes, ...) the second level corresponds to the year and the third level to the Calendar Week. Right now Notes sorts this really awkwardly as the second column doesn't sort it like "CW1/CW11/CW12.../CW2/CW21/...CW3/CW31/CW32". A viable option for me would be to choose the sorting style, e.g. by folder name or creation date. Another option would be for me would be to sort numbers automatically in ascending order with respect to all digits (1,2,3...,11,12,13...,20,21,22...)

[CorneliousJD]

I concur here - we desperately need better handling of subfolders, it makes using this cumbersome when you have nested folders.

[CorneliousJD]

To add to this - the Android app handles this perfectly - see here - nextcloud/notes-android#1154