Add unit tests

- TransferService tests for linked transfer entries
- YearlyStatement calculation tests for balance verification

tests/Duempelkas.Tests/Duempelkas.Tests.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <TargetFramework>net10.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+    <IsPackable>false</IsPackable>
+    <IsTestProject>true</IsTestProject>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="*" />
+    <PackageReference Include="xunit" Version="*" />
+    <PackageReference Include="xunit.runner.visualstudio" Version="*">
+      <PrivateAssets>all</PrivateAssets>
+      <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
+    </PackageReference>
+    <PackageReference Include="FluentAssertions" Version="*" />
+    <PackageReference Include="Microsoft.EntityFrameworkCore.Sqlite" Version="10.0.*" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\..\src\Duempelkas.Domain\Duempelkas.Domain.csproj" />
+    <ProjectReference Include="..\..\src\Duempelkas.App\Duempelkas.App.csproj" />
+    <ProjectReference Include="..\..\src\Duempelkas.Infrastructure\Duempelkas.Infrastructure.csproj" />
+  </ItemGroup>
+</Project>

tests/Duempelkas.Tests/TransferServiceTests.cs

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+using Duempelkas.Domain.Entities;
+using Duempelkas.Domain.Enums;
+using Duempelkas.Infrastructure.Persistence;
+using Duempelkas.Infrastructure.Services;
+using FluentAssertions;
+using Microsoft.EntityFrameworkCore;
+using Xunit;
+
+namespace Duempelkas.Tests;
+
+public class TransferServiceTests : IDisposable
+{
+    private readonly FinanceDbContext _db;
+    private readonly EntryService _entryService;
+    private readonly BalanceQueryService _balanceQueryService;
+
+    public TransferServiceTests()
+    {
+        var options = new DbContextOptionsBuilder<FinanceDbContext>()
+            .UseSqlite("Data Source=:memory:")
+            .Options;
+
+        _db = new FinanceDbContext(options);
+        _db.Database.OpenConnection();
+        _db.Database.EnsureCreated();
+
+        _entryService = new EntryService(_db);
+        _balanceQueryService = new BalanceQueryService(_db);
+    }
+
+    [Fact]
+    public async Task CreateTransfer_CreatesLinkedExpenseAndIncome()
+    {
+        var accountA = new Account { Name = "Account A" };
+        var accountB = new Account { Name = "Account B" };
+        _db.Accounts.AddRange(accountA, accountB);
+        await _db.SaveChangesAsync();
+
+        await _entryService.CreateTransferAsync(accountA.Id, accountB.Id, new DateTime(2026, 3, 15), "Test Transfer", 100.00m);
+
+        var entries = await _db.Entries.ToListAsync();
+        entries.Should().HaveCount(2);
+
+        var sourceEntry = entries.Single(e => e.AccountId == accountA.Id);
+        sourceEntry.Type.Should().Be(EntryType.Expense);
+        sourceEntry.Amount.Should().Be(100.00m);
+        sourceEntry.Title.Should().Be("Test Transfer");
+        sourceEntry.DisplayId.Should().Be("2026-001");
+
+        var targetEntry = entries.Single(e => e.AccountId == accountB.Id);
+        targetEntry.Type.Should().Be(EntryType.Income);
+        targetEntry.Amount.Should().Be(100.00m);
+        targetEntry.DisplayId.Should().Be("2026-002");
+
+        var links = await _db.TransferLinks.ToListAsync();
+        links.Should().HaveCount(1);
+        links[0].SourceEntryId.Should().Be(sourceEntry.Id);
+        links[0].TargetEntryId.Should().Be(targetEntry.Id);
+
+        var balanceA = await _balanceQueryService.GetAccountBalanceAsync(accountA.Id);
+        balanceA.TotalBalance.Should().Be(-100.00m);
+
+        var balanceB = await _balanceQueryService.GetAccountBalanceAsync(accountB.Id);
+        balanceB.TotalBalance.Should().Be(100.00m);
+    }
+
+    [Fact]
+    public async Task DeleteTransfer_SoftDeletesBothSides()
+    {
+        var accountA = new Account { Name = "Account A" };
+        var accountB = new Account { Name = "Account B" };
+        _db.Accounts.AddRange(accountA, accountB);
+        await _db.SaveChangesAsync();
+
+        await _entryService.CreateTransferAsync(accountA.Id, accountB.Id, new DateTime(2026, 3, 15), "Test Transfer", 100.00m);
+        var sourceEntry = await _db.Entries.FirstAsync(e => e.AccountId == accountA.Id);
+
+        await _entryService.DeleteEntryAsync(sourceEntry.Id);
+
+        var entries = await _db.Entries.ToListAsync();
+        entries.Should().HaveCount(2);
+        entries.Should().AllSatisfy(e => e.IsDeleted.Should().BeTrue());
+
+        var links = await _db.TransferLinks.ToListAsync();
+        links.Should().HaveCount(1);
+
+        var balanceA = await _balanceQueryService.GetAccountBalanceAsync(accountA.Id);
+        balanceA.TotalBalance.Should().Be(0m);
+
+        var balanceB = await _balanceQueryService.GetAccountBalanceAsync(accountB.Id);
+        balanceB.TotalBalance.Should().Be(0m);
+    }
+
+    [Fact]
+    public async Task CreateTransfer_SameAccount_Throws()
+    {
+        var account = new Account { Name = "Account A" };
+        _db.Accounts.Add(account);
+        await _db.SaveChangesAsync();
+
+        var act = () => _entryService.CreateTransferAsync(account.Id, account.Id, DateTime.Today, "Bad Transfer", 50m);
+        await act.Should().ThrowAsync<InvalidOperationException>();
+    }
+
+    public void Dispose()
+    {
+        _db.Database.CloseConnection();
+        _db.Dispose();
+    }
+}

tests/Duempelkas.Tests/YearlyStatementCalculationTests.cs

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+using Duempelkas.Domain.Entities;
+using Duempelkas.Domain.Enums;
+using Duempelkas.Infrastructure.Persistence;
+using Duempelkas.Infrastructure.Services;
+using FluentAssertions;
+using Microsoft.EntityFrameworkCore;
+using Xunit;
+
+namespace Duempelkas.Tests;
+
+public class BalanceCalculationTests : IDisposable
+{
+    private readonly FinanceDbContext _db;
+    private readonly BalanceQueryService _balanceQueryService;
+    private readonly EntryService _entryService;
+
+    public BalanceCalculationTests()
+    {
+        var options = new DbContextOptionsBuilder<FinanceDbContext>()
+            .UseSqlite("Data Source=:memory:")
+            .Options;
+
+        _db = new FinanceDbContext(options);
+        _db.Database.OpenConnection();
+        _db.Database.EnsureCreated();
+
+        _balanceQueryService = new BalanceQueryService(_db);
+        _entryService = new EntryService(_db);
+    }
+
+    [Fact]
+    public async Task AccountBalance_CalculatesCorrectTotals()
+    {
+        var account = new Account { Name = "Test Account", CarryoverBalance = 500.00m };
+        _db.Accounts.Add(account);
+        await _db.SaveChangesAsync();
+
+        _db.Entries.AddRange(
+            new Entry { AccountId = account.Id, DisplayId = "2026-001", Type = EntryType.Income, Date = new DateTime(2026, 1, 15), Title = "Gehalt", Amount = 800.00m },
+            new Entry { AccountId = account.Id, DisplayId = "2026-002", Type = EntryType.Income, Date = new DateTime(2026, 2, 15), Title = "Bonus", Amount = 400.00m },
+            new Entry { AccountId = account.Id, DisplayId = "2026-003", Type = EntryType.Expense, Date = new DateTime(2026, 1, 20), Title = "Miete", Amount = 200.00m },
+            new Entry { AccountId = account.Id, DisplayId = "2026-004", Type = EntryType.Expense, Date = new DateTime(2026, 2, 20), Title = "Lebensmittel", Amount = 250.00m }
+        );
+        await _db.SaveChangesAsync();
+
+        var balance = await _balanceQueryService.GetAccountBalanceAsync(account.Id);
+
+        balance.CarryoverBalance.Should().Be(500.00m);
+        balance.TotalIncome.Should().Be(1200.00m);
+        balance.TotalExpense.Should().Be(450.00m);
+        balance.TotalBalance.Should().Be(1250.00m);
+    }
+
+    [Fact]
+    public async Task AccountBalance_IncludesTransfersCorrectly()
+    {
+        var accountA = new Account { Name = "Account A", CarryoverBalance = 500.00m };
+        var accountB = new Account { Name = "Account B" };
+        _db.Accounts.AddRange(accountA, accountB);
+        await _db.SaveChangesAsync();
+
+        _db.Entries.Add(new Entry
+        {
+            AccountId = accountA.Id,
+            DisplayId = "2026-001",
+            Type = EntryType.Income,
+            Date = new DateTime(2026, 1, 10),
+            Title = "Gehalt",
+            Amount = 1000.00m
+        });
+        await _db.SaveChangesAsync();
+
+        await _entryService.CreateTransferAsync(accountA.Id, accountB.Id, new DateTime(2026, 2, 1), "Umbuchung nach B", 300.00m);
+
+        var balanceA = await _balanceQueryService.GetAccountBalanceAsync(accountA.Id);
+
+        balanceA.CarryoverBalance.Should().Be(500.00m);
+        balanceA.TotalIncome.Should().Be(1000.00m);
+        balanceA.TotalExpense.Should().Be(300.00m);
+        balanceA.TotalBalance.Should().Be(1200.00m);
+    }
+
+    [Fact]
+    public async Task AccountBalance_WithCarryover()
+    {
+        var account = new Account { Name = "Konto mit Übertrag", CarryoverBalance = 1000.00m };
+        _db.Accounts.Add(account);
+        await _db.SaveChangesAsync();
+
+        _db.Entries.AddRange(
+            new Entry { AccountId = account.Id, DisplayId = "2025-001", Type = EntryType.Income, Date = new DateTime(2025, 6, 1), Title = "Einnahme 2025", Amount = 500.00m },
+            new Entry { AccountId = account.Id, DisplayId = "2026-001", Type = EntryType.Income, Date = new DateTime(2026, 6, 1), Title = "Einnahme 2026", Amount = 300.00m },
+            new Entry { AccountId = account.Id, DisplayId = "2026-002", Type = EntryType.Expense, Date = new DateTime(2026, 7, 1), Title = "Ausgabe 2026", Amount = 150.00m }
+        );
+        await _db.SaveChangesAsync();
+
+        var balance = await _balanceQueryService.GetAccountBalanceAsync(account.Id);
+
+        // 1000 + 500 + 300 - 150 = 1650
+        balance.TotalBalance.Should().Be(1650.00m);
+        balance.CarryoverBalance.Should().Be(1000.00m);
+    }
+
+    public void Dispose()
+    {
+        _db.Database.CloseConnection();
+        _db.Dispose();
+    }
+}