refactor(P5b-1): CommunityToolkit.Mvvm 도입 + OptionsViewModel 추출 (옵션 로직 테스트화)

MVVM 첫 슬라이스 — 테스트 불가능했던 코드비하인드 BuildOptions의 순수 옵션 구성 로직을
헤드리스 단위테스트 가능한 OptionsViewModel.ToConvertOptions로 추출. XAML 바인딩 미변경(런타임 위험 0).

- CommunityToolkit.Mvvm 8.2.2(CPM) + App.csproj. App/ViewModels/OptionsViewModel.cs:
  ObservableObject + [ObservableProperty](Quality/CustomOutputDirectory/ConflictRule/AiTaskIndex/
  TargetLanguage/VideoPreferGpu) → 향후 옵션 패널 직접 바인딩 토대. ToConvertOptions()는 순수.
- MainWindow.BuildOptions: 컨트롤 값을 VM에 반영 후 ToConvertOptions() 위임(동작 동일).
- Tests → App ProjectReference 추가(App 뷰모델 헤드리스 테스트 가능화).
- OptionsViewModelTests 10케이스(품질매핑·AVIF clamp·커스텀폴더 trim·AI작업·대상언어·충돌/GPU).

71 → 81개 테스트 전부 그린, 빌드 0경고/0오류.

src/Everything2Everything.Tests/Everything2Everything.Tests.csproj

@@ -18,6 +18,7 @@
 
   <ItemGroup>
     <ProjectReference Include="..\Everything2Everything.Core\Everything2Everything.Core.csproj" />
+    <ProjectReference Include="..\Everything2Everything.App\Everything2Everything.App.csproj" />
   </ItemGroup>
 
 </Project>

src/Everything2Everything.Tests/OptionsViewModelTests.cs

@@ -0,0 +1,68 @@
+using Everything2Everything.App.ViewModels;
+using Everything2Everything.Core;
+using Xunit;
+
+namespace Everything2Everything.Tests;
+
+/// <summary>
+/// OptionsViewModel.ToConvertOptions 단위 테스트 — 기존 MainWindow.BuildOptions(코드비하인드, 테스트 불가)에서
+/// 추출한 순수 옵션 구성 로직을 헤드리스로 검증(P5b MVVM). UI 컨트롤 읽기와 분리되어 회귀를 기계적으로 방어.
+/// </summary>
+public class OptionsViewModelTests
+{
+    [Fact]
+    public void ToConvertOptions_MapsQualityToImageFormats()
+    {
+        var o = new OptionsViewModel { Quality = 70 }.ToConvertOptions();
+        Assert.Equal(70, o.Jpeg.Quality);
+        Assert.Equal(70, o.Webp.Quality);
+        Assert.Equal(40, o.Avif.Quality); // AVIF = Quality - 30
+    }
+
+    [Fact]
+    public void ToConvertOptions_AvifQualityClampedToFloor()
+    {
+        Assert.Equal(1, new OptionsViewModel { Quality = 10 }.ToConvertOptions().Avif.Quality);
+    }
+
+    [Fact]
+    public void ToConvertOptions_EmptyCustomDir_UsesSubfolder()
+    {
+        var o = new OptionsViewModel { CustomOutputDirectory = "   " }.ToConvertOptions();
+        Assert.Equal(OutputLocation.SubfolderBesideSource, o.OutputLocation);
+        Assert.Null(o.CustomOutputDirectory);
+    }
+
+    [Fact]
+    public void ToConvertOptions_CustomDir_TrimmedAndCustomLocation()
+    {
+        var o = new OptionsViewModel { CustomOutputDirectory = "  C:\\out  " }.ToConvertOptions();
+        Assert.Equal(OutputLocation.Custom, o.OutputLocation);
+        Assert.Equal("C:\\out", o.CustomOutputDirectory);
+    }
+
+    [Theory]
+    [InlineData(0, "summarize")]
+    [InlineData(1, "translate")]
+    [InlineData(2, "proofread")]
+    [InlineData(99, "summarize")]
+    public void ToConvertOptions_MapsAiTask(int index, string expected)
+    {
+        Assert.Equal(expected, new OptionsViewModel { AiTaskIndex = index }.ToConvertOptions().Ai.Task);
+    }
+
+    [Fact]
+    public void ToConvertOptions_TargetLanguage_EmptyBecomesNullElseTrimmed()
+    {
+        Assert.Null(new OptionsViewModel { TargetLanguage = "   " }.ToConvertOptions().Ai.TargetLanguage);
+        Assert.Equal("일본어", new OptionsViewModel { TargetLanguage = " 일본어 " }.ToConvertOptions().Ai.TargetLanguage);
+    }
+
+    [Fact]
+    public void ToConvertOptions_PassesConflictAndGpu()
+    {
+        var o = new OptionsViewModel { ConflictRule = NameCollision.Skip, VideoPreferGpu = false }.ToConvertOptions();
+        Assert.Equal(NameCollision.Skip, o.OnCollision);
+        Assert.False(o.VideoPreferGpu);
+    }
+}