[Feature request] Hierarchical bookmarks

[squarfed]

**Checklist

• I make sure that there are no existing issues - open or closed - which I could contribute my information to.
• I believe this feature request will benefit more than 10% of users, not just myself.

---

** Description
The bookmark system has a flat structure, containing all playlists at top level. This makes bookmarks unmanageable as soon as there are more than a few playlist.
The request is for a hierarchical system where a playlist can contain a playlist.

[github-actions]

This issue does not follow the template and will be closed. Please update the issue following the template and reopen it.

[InfinityLoop1308]

After consideration, we believe that nested bookmark playlists do not align with the app's design, so we have no intention of implementing this feature. Regarding the filtering issue after the playlist reaches a certain scale, we will introduce search and sorting to solve this problem.