Refactor. Add data layer

BudgetApp.Data/Entities/Category.cs

@@ -0,0 +1,23 @@
+using System.ComponentModel.DataAnnotations;
+
+namespace BudgetApp.Data.Entities;
+
+public class Category
+{
+    public int Id { get; set; }
+
+    [Required, MaxLength(100)]
+    public string Name { get; set; } = string.Empty;
+
+    [MaxLength(10)]
+    public string Icon { get; set; } = string.Empty;
+
+    [Required]
+    public string UserId { get; set; } = string.Empty;
+
+    public ApplicationUser User { get; set; } = default!;
+
+    public bool IsDefault { get; set; }
+
+    public List<Transaction> Transactions { get; set; } = [];
+}