Refactor. Add data layer

BudgetApp.Data/Interfaces/IAccountRepository.cs

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+using BudgetApp.Data.Entities;
+
+namespace BudgetApp.Data.Interfaces;
+
+public interface IAccountRepository
+{
+    Task<List<Account>> GetAccountsAsync(string userId);
+    Task<Account?> GetAccountAsync(int id, string userId);
+    Task CreateAccountAsync(Account account);
+    Task UpdateAccountAsync(Account account);
+    Task UpdateBalanceAsync(int id, string userId, decimal newBalance);
+    Task UpdateAccountDetailsAsync(int id, string userId, string name, string color, string currencyCode);
+    Task DeleteAccountAsync(int id, string userId);
+}

BudgetApp.Data/Interfaces/ICategoryRepository.cs

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+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);
+}

BudgetApp.Data/Interfaces/ISalaryRepository.cs

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+using BudgetApp.Data.Entities;
+
+namespace BudgetApp.Data.Interfaces;
+
+public interface ISalaryRepository
+{
+    Task<List<Salary>> GetSalariesAsync(string userId);
+    Task CreateSalaryAsync(Salary salary, DateTime windowEnd);
+    Task DeleteSalaryAsync(int id, string userId);
+    Task MaterializeSalaryTransactionsAsync(string userId, DateTime windowEnd);
+    Task<DateTime?> GetNextPaydateAsync(int salaryId);
+}

BudgetApp.Data/Interfaces/ITransactionRepository.cs

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+using BudgetApp.Data.Entities;
+
+namespace BudgetApp.Data.Interfaces;
+
+public interface ITransactionRepository
+{
+    Task<List<Transaction>> GetTransactionsAsync(string userId);
+    Task<List<Transaction>> GetRecentTransactionsAsync(string userId, int count = 5);
+    Task<List<Transaction>> GetUpcomingTransactionsAsync(string userId, int count = 5);
+    Task<List<Transaction>> GetTransactionsInRangeAsync(string userId, DateTime from, DateTime to);
+    Task<List<Transaction>> GetTransactionsByAccountAsync(int accountId, string userId);
+    Task CreateTransactionAsync(Transaction transaction);
+    Task UpdateTransactionAsync(int id, string userId, string description, decimal amount, DateTime date,
+        TransactionType type, int accountId, int? categoryId,
+        RecurrenceType recurrenceType = RecurrenceType.None, DateTime? recurrenceEndDate = null);
+    Task DeleteTransactionAsync(int id, string userId);
+    Task DeleteTransactionSeriesAsync(int id, string userId);
+    Task MarkAsPaidAsync(int id, string userId, bool isPaid = true);
+    Task MaterializeRecurringTransactionsAsync(string userId, DateTime? windowEnd = null);
+}