Directory Traversal & Filesystem can be accessed by a non-admin user

Swing Music is a self-hosted music player for local audio files. Prior to version 2.1.4, Swing Music's `list_folders()` function in the `/folder/dir-browser` endpoint is vulnerable to directory traversal attacks. Any authenticated user (including non-admin) can browse arbitrary directories on the server filesystem. Version 2.1.4 fixes the issue.