Refactor. Add data layer

BudgetApp.Data/Entities/Account.cs

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+using System.ComponentModel.DataAnnotations;
+
+namespace BudgetApp.Data.Entities;
+
+public class Account
+{
+    public int Id { get; set; }
+
+    [Required, MaxLength(100)]
+    public string Name { get; set; } = string.Empty;
+
+    public decimal CurrentBalance { get; set; }
+
+    [Required, MaxLength(7)]
+    public string Color { get; set; } = "#1b6ec2";
+
+    [Required, MaxLength(3)]
+    public string CurrencyCode { get; set; } = "GBP";
+
+    [Required]
+    public string UserId { get; set; } = string.Empty;
+
+    public ApplicationUser User { get; set; } = default!;
+
+    public List<Transaction> Transactions { get; set; } = [];
+
+    public static readonly Dictionary<string, string> CurrencySymbols = new()
+    {
+        ["USD"] = "$", ["EUR"] = "€", ["GBP"] = "£", ["JPY"] = "¥",
+        ["CAD"] = "C$", ["AUD"] = "A$", ["CHF"] = "CHF", ["CNY"] = "¥",
+        ["INR"] = "₹", ["BRL"] = "R$", ["KRW"] = "₩", ["MXN"] = "MX$",
+        ["SEK"] = "kr", ["NOK"] = "kr", ["DKK"] = "kr", ["NZD"] = "NZ$",
+        ["ZAR"] = "R", ["PLN"] = "zł", ["TRY"] = "₺", ["THB"] = "฿"
+    };
+
+    public string GetSymbol() => CurrencySymbols.GetValueOrDefault(CurrencyCode, CurrencyCode);
+
+    public string FormatAmount(decimal amount) => $"{GetSymbol()}{amount:N2}";
+}

BudgetApp.Data/Entities/ApplicationUser.cs

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+using Microsoft.AspNetCore.Identity;
+
+namespace BudgetApp.Data.Entities;
+
+public class ApplicationUser : IdentityUser
+{
+}

BudgetApp.Data/Entities/Category.cs

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+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; } = [];
+}

BudgetApp.Data/Entities/RecurrenceType.cs

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+namespace BudgetApp.Data.Entities;
+
+public enum RecurrenceType
+{
+    None,
+    Daily,
+    Weekly,
+    Monthly,
+    Yearly
+}

BudgetApp.Data/Entities/Salary.cs

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+using System.ComponentModel.DataAnnotations;
+
+namespace BudgetApp.Data.Entities;
+
+public class Salary
+{
+    public int Id { get; set; }
+
+    [Required, MaxLength(200)]
+    public string Name { get; set; } = string.Empty;
+
+    public decimal Amount { get; set; }
+
+    public RecurrenceType RecurrenceType { get; set; } = RecurrenceType.Monthly;
+
+    public DateTime StartDate { get; set; } = DateTime.Today;
+
+    public int AccountId { get; set; }
+    public Account Account { get; set; } = default!;
+
+    public int? CategoryId { get; set; }
+    public Category? Category { get; set; }
+
+    [Required]
+    public string UserId { get; set; } = string.Empty;
+    public ApplicationUser User { get; set; } = default!;
+
+    public string GetPaydaySummary() => RecurrenceType switch
+    {
+        RecurrenceType.Weekly  => $"Every week on {StartDate:dddd}",
+        RecurrenceType.Monthly => $"Monthly on the {GetOrdinal(StartDate.Day)}",
+        RecurrenceType.Yearly  => $"Annually on {StartDate:MMMM d}",
+        _                      => RecurrenceType.ToString()
+    };
+
+    private static string GetOrdinal(int day) => day switch
+    {
+        1 or 21 or 31 => $"{day}st",
+        2 or 22        => $"{day}nd",
+        3 or 23        => $"{day}rd",
+        _              => $"{day}th"
+    };
+}

BudgetApp.Data/Entities/Transaction.cs

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+using System.ComponentModel.DataAnnotations;
+
+namespace BudgetApp.Data.Entities;
+
+public class Transaction
+{
+    public int Id { get; set; }
+
+    [Required, MaxLength(200)]
+    public string Description { get; set; } = string.Empty;
+
+    public decimal Amount { get; set; }
+
+    public DateTime Date { get; set; } = DateTime.Today;
+
+    public TransactionType Type { get; set; } = TransactionType.Expense;
+
+    public RecurrenceType RecurrenceType { get; set; } = RecurrenceType.None;
+
+    public DateTime? RecurrenceEndDate { get; set; }
+
+    public int? ParentTransactionId { get; set; }
+    public Transaction? ParentTransaction { get; set; }
+
+    public int AccountId { get; set; }
+    public Account Account { get; set; } = default!;
+
+    public int? CategoryId { get; set; }
+    public Category? Category { get; set; }
+
+    public bool IsPaid { get; set; } = false;
+
+    public int? SalaryId { get; set; }
+    public Salary? Salary { get; set; }
+}

BudgetApp.Data/Entities/TransactionType.cs

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+namespace BudgetApp.Data.Entities;
+
+public enum TransactionType
+{
+    Income,
+    Expense
+}