Added initial audio library logic.

Harmonia.Core/Library/Folder.cs

@@ -0,0 +1,16 @@
+namespace Harmonia.Core.Library;
+
+public class Folder
+{
+    public int FolderId { get; set; }
+
+    public int? ParentFolderId { get; set; }
+    public Folder? ParentFolder { get; set; }
+
+    public required string Name { get; set; }
+
+    public int LocationId { get; set; }
+    public required Location Location { get; set; }
+
+    public virtual ICollection<Song> Songs { get; set; } = [];
+}