Adding `Folders` for clubbing notes for similar topics

### What
We could add `Folder` for clubbing notes related to one subject but different topics in a folder. And if possible we could make it continous like creating as many folders inside a folders and going on deeper.

### Why
There are many notes files which can be clubbed under a single folder, the tag option which is provided can be used for clubbing notes but a `folder` would be a good option as it would make it much cleaner. Another reason for adding folder is that we can make a downard chain, for ex;- in a subject we have 2 modules so we can create a sub folder inside the main and folder and inside that subfolder we could make notes for module 1 and likewise for module 2.
### How
n/a

The issue names no files, tests, or entry points. Start by reviewing how notes are currently organized and how tags and navigation work in the TypeScript/Electron app; clarify folder creation, nesting, and migration expectations before implementation. Done should include agreed behavior and coverage for the supported folder hierarchy.

Written by the indexing model from the issue text.