Refactor. Add data layer

BudgetApp.Data/Interfaces/ICategoryRepository.cs

@@ -0,0 +1,12 @@
+using BudgetApp.Data.Entities;
+
+namespace BudgetApp.Data.Interfaces;
+
+public interface ICategoryRepository
+{
+    Task<List<Category>> GetCategoriesAsync(string userId);
+    Task EnsureDefaultCategoriesAsync(string userId);
+    Task CreateCategoryAsync(Category category);
+    Task UpdateCategoryAsync(int id, string userId, string name, string icon);
+    Task DeleteCategoryAsync(int id, string userId);
+}