refactor(P5a): App DI 컴포지션 루트 + MainWindow 생성자 주입 (서비스로케이터 제거)

MVVM(P5b)의 선행조건 — 엔진/설정을 생성자 주입으로 가시화하고 서비스로케이터 결합을 제거.

- App.xaml.cs: new DpapiSettingsStore()+CreateDefault() 직접 생성 → ServiceCollection.AddEverything2Everything()
  (P2 DI 재사용) 컴포지션 루트. Settings/Engine은 컨테이너 단일 싱글턴에서 해소(공유 보존).
  Generic Host 대신 ServiceProvider 사용 — Environment.Exit 진입점과 라이프사이클 충돌 회피(적대적 리뷰 반영).
- MainWindow: ((App)Application.Current).Engine/.Settings 서비스로케이터 8곳 → 생성자 주입 _engine/_settings.
  App.ShowMainWindow/ShowConvertDialog가 주입. 헤드리스 테스트 가능성 토대.

70개 테스트 전부 그린, 빌드 0경고/0오류.

src/Everything2Everything.App/App.xaml.cs

@@ -2,19 +2,27 @@ using System.Windows;
 using Everything2Everything.App.Cli;
 using Everything2Everything.App.Views;
 using Everything2Everything.Core;
+using Microsoft.Extensions.DependencyInjection;
 
 namespace Everything2Everything.App;
 
 public partial class App : Application
 {
-    /// <summary>App·LlmProvider가 공유하는 설정 저장소 (키 저장 즉시 변환에 반영).</summary>
-    public ISettingsStore Settings { get; } = new DpapiSettingsStore();
+    private readonly IServiceProvider _services;
+
+    /// <summary>App·LlmProvider가 공유하는 설정 저장소 (키 저장 즉시 변환에 반영). DI 컨테이너 단일 싱글턴.</summary>
+    public ISettingsStore Settings { get; }
 
     public ConversionEngine Engine { get; }
 
     public App()
     {
-        Engine = Everything2EverythingBootstrap.CreateDefault(Settings);
+        // 컴포지션 루트 — Core의 DI 확장(Scrutor 자동등록)으로 Provider/Registry/Engine/Settings를 구성.
+        var services = new ServiceCollection();
+        services.AddEverything2Everything();
+        _services = services.BuildServiceProvider();
+        Settings = _services.GetRequiredService<ISettingsStore>();
+        Engine = _services.GetRequiredService<ConversionEngine>();
     }
 
     protected override async void OnStartup(StartupEventArgs e)
@@ -68,14 +76,14 @@ public partial class App : Application
 
     private void ShowMainWindow()
     {
-        var window = new MainWindow();
+        var window = new MainWindow(Engine, Settings);
         MainWindow = window;
         window.Show();
     }
 
     private void ShowConvertDialog(IReadOnlyList<string> files)
     {
-        var window = new Views.MainWindow(files);
+        var window = new Views.MainWindow(Engine, Settings, files);
         MainWindow = window;
         window.Show();
     }

src/Everything2Everything.App/Views/MainWindow.xaml.cs

@@ -16,6 +16,8 @@ namespace Everything2Everything.App.Views;
 
 public partial class MainWindow : Wpf.Ui.Controls.FluentWindow
 {
+    private readonly ConversionEngine _engine;
+    private readonly ISettingsStore _settings;
     private readonly ObservableCollection<QueueItem> _activeQueue = new();
     private readonly ObservableCollection<DateGroup> _pastResults = new();
     private CancellationTokenSource? _cts;
@@ -28,10 +30,11 @@ public partial class MainWindow : Wpf.Ui.Controls.FluentWindow
     public ICommand CloseCommand { get; }
     public ICommand RefreshCommand { get; }
 
-    public MainWindow() : this(null) { }
-
-    public MainWindow(IReadOnlyList<string>? initialFiles)
+    public MainWindow(ConversionEngine engine, ISettingsStore settings, IReadOnlyList<string>? initialFiles = null)
     {
+        _engine = engine;
+        _settings = settings;
+
         AddFilesCommand = new RelayCommand(_ => PickAndAddFiles());
         ProcessQueueCommand = new RelayCommand(_ => OnProcessQueueClick(this, new RoutedEventArgs()),
             _ => _activeQueue.Count > 0 && _cts is null);
@@ -66,7 +69,7 @@ public partial class MainWindow : Wpf.Ui.Controls.FluentWindow
 
     private async Task RefreshCapabilityStatusAsync()
     {
-        var engine = ((App)Application.Current).Engine;
+        var engine = _engine;
         var notReady = new List<string>();
         foreach (var p in engine.Providers.All)
         {
@@ -90,7 +93,7 @@ public partial class MainWindow : Wpf.Ui.Controls.FluentWindow
 
     private void OnSettingsClick(object sender, RoutedEventArgs e)
     {
-        var win = new SettingsWindow(((App)Application.Current).Settings) { Owner = this };
+        var win = new SettingsWindow(_settings) { Owner = this };
         win.ShowDialog();
         _ = RefreshCapabilityStatusAsync();
         RefreshAvailableOutputFormats();
@@ -304,7 +307,7 @@ public partial class MainWindow : Wpf.Ui.Controls.FluentWindow
         var targetLang = AiTargetLangBox?.Text?.Trim();
         opts.Ai.TargetLanguage = string.IsNullOrEmpty(targetLang) ? null : targetLang;
 
-        opts.VideoPreferGpu = ((App)Application.Current).Settings.Get("video.gpu") != "false";
+        opts.VideoPreferGpu = _settings.Get("video.gpu") != "false";
 
         return opts;
     }
@@ -563,7 +566,7 @@ public partial class MainWindow : Wpf.Ui.Controls.FluentWindow
         UpdateProcessQueueButton();
         ShowProcessingProgress(snapshot.Count);
 
-        var engine = ((App)Application.Current).Engine;
+        var engine = _engine;
         var options = BuildOptions();
 
         var reporter = new Progress<ConvertProgress>(p =>
@@ -780,7 +783,7 @@ public partial class MainWindow : Wpf.Ui.Controls.FluentWindow
     {
         try
         {
-            ContextMenuRegistrar.Register(((App)Application.Current).Engine);
+            ContextMenuRegistrar.Register(_engine);
             MessageBox.Show(this,
                 "컨텍스트 메뉴를 등록했습니다.\n파일 우클릭 → \"추가 옵션 표시\" 또는 \"JPEG로 빠른 변환/변환…\".",
                 "Everything2Everything", MessageBoxButton.OK, MessageBoxImage.Information);
@@ -794,7 +797,7 @@ public partial class MainWindow : Wpf.Ui.Controls.FluentWindow
 
     private void OnDiagnoseClick(object sender, RoutedEventArgs e)
     {
-        var window = new DiagnoseWindow(((App)Application.Current).Engine) { Owner = this };
+        var window = new DiagnoseWindow(_engine) { Owner = this };
         window.ShowDialog();
     }
 
@@ -934,7 +937,7 @@ public partial class MainWindow : Wpf.Ui.Controls.FluentWindow
     private LossClass? WorstLossForQueue(string outputExt)
     {
         if (_activeQueue.Count == 0) return null;
-        var graph = ((App)Application.Current).Engine.Providers.Graph;
+        var graph = _engine.Providers.Graph;
         LossClass? worst = null;
         foreach (var item in _activeQueue)
         {
@@ -971,7 +974,7 @@ public partial class MainWindow : Wpf.Ui.Controls.FluentWindow
     {
         if (OutputFormatCombo is null) return;
 
-        var engine = ((App)Application.Current).Engine;
+        var engine = _engine;
         IReadOnlyCollection<string> available;
 
         if (_activeQueue.Count == 0)