T3-11/14/15 + M2 잔여: 권한 정책 UI(PermissionGate/PermissionRequested 다이얼로그+SavesInProfile), 추적 방지(TrackerBlocklist/WebResourceRequested 차단+실드 배지·팝업), 주소창 진행 바+파라미터 제거 복사(UrlCleaner/NavProgressTracker), U5 완료 알림(Mobile.Core 감지기+Plugin.LocalNotification), 재생중 ongoing 미디어 알림, 썸네일/해상도 API(Thumbnailer 사이드카 캐시+/thumb)+라이브러리 그리드, 외부망 안내(ConnectionTroubleshoot). 147 tests GREEN

VideoDownloader.Tests/Server/RemoteServerE2ETests.cs

@@ -155,6 +155,44 @@ public class RemoteServerE2ETests : IDisposable
         Assert.Equal(93.5, persisted!.PositionSec);
     }
 
+    [Fact]
+    public async Task 썸네일_API_E2E_생성_캐시_404()
+    {
+        Directory.CreateDirectory(_libDir);
+        var mediaPath = Path.Combine(_libDir, "clip.mp4");
+        File.WriteAllBytes(mediaPath, new byte[2048]);
+
+        // 가짜 ffmpeg: 출력 경로에 JPEG 생성
+        var fake = new Tests.Platform.FakeProcessRunner
+        {
+            Behavior = (spec, _, _) =>
+            {
+                File.WriteAllBytes(spec.Args[^1], new byte[] { 0xFF, 0xD8, 0xFF, 0xE0, 0x01 });
+                return Task.FromResult(new VideoDownloader.Core.Platform.ProcessResult(0, "", ""));
+            },
+        };
+
+        await using var server = await ServerHost.StartAsync(new ServerOptions
+        {
+            Port = 0, Token = "thumb-token", LibraryFolder = _libDir,
+            Queue = new FakeQueue(),
+            Thumbnailer = new Thumbnailer(fake),
+        });
+        using var http = new HttpClient { BaseAddress = new Uri($"http://127.0.0.1:{server.Port}") };
+
+        // 1) 익명 401
+        Assert.Equal(System.Net.HttpStatusCode.Unauthorized, (await http.GetAsync("/api/library/xyz/thumb")).StatusCode);
+
+        // 2) 없는 항목 404
+        http.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Bearer", "thumb-token");
+        Assert.Equal(System.Net.HttpStatusCode.NotFound, (await http.GetAsync("/api/library/xyz/thumb")).StatusCode);
+
+        // 3) 실제 항목 → 200 + JPEG 매직
+        var lib = (await http.GetFromJsonAsync<List<LibraryItemDto>>("/api/library"))!;
+        var thumb = await http.GetByteArrayAsync($"/api/library/{lib[0].Id}/thumb");
+        Assert.True(thumb.Length >= 2 && thumb[0] == 0xFF && thumb[1] == 0xD8, "JPEG 매직 바이트");
+    }
+
     public void Dispose()
     {
         try { Directory.Delete(_libDir, true); } catch { }