feat: first-class SponsorBlock toggle

A "Remove sponsor segments" switch next to "Split by chapters" queues
the download with the same postprocessor pair the CLI's
--sponsorblock-remove sponsor builds (SponsorBlock + ModifyChapters).
The flag persists as a cookie like the other form options, survives in
the queue records, and is carried into retries.

The pair is registered above the chapter-splitting block: yt-dlp runs
same-stage postprocessors in list order, so ModifyChapters has to
rewrite the chapter list before FFmpegSplitChapters cuts the file up,
matching what the CLI builds for --sponsorblock-remove sponsor
--split-chapters. With both toggles on the other way around the chapter
files keep the sponsor segments and the removal desyncs the remaining
chapter timings.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

app/tests/test_download_queue.py

@@ -476,6 +476,32 @@ async def test_retry_keeps_overrides_while_still_allowed(dq_env):
     assert dq.queue.get(url).info.ytdl_options_overrides == {"writesubtitles": True}
 
 
+@pytest.mark.asyncio
+async def test_retry_carries_the_sponsorblock_flag(dq_env):
+    notifier = AsyncMock()
+    dq = DownloadQueue(dq_env, notifier)
+    url = "https://example.com/watch?v=1"
+    dq.done.put(
+        Download(None, None, None, None, "best", "any", {}, _failed_playlist_item(url, sponsorblock=True))
+    )
+
+    def fake_extract(self, extracted_url, *_args, **_kwargs):
+        return {
+            "_type": "video",
+            "id": "vid1",
+            "title": "Test Video",
+            "url": extracted_url,
+            "webpage_url": extracted_url,
+        }
+
+    with patch.object(DownloadQueue, "_DownloadQueue__extract_info", fake_extract), \
+         patch.object(DownloadQueue, "_DownloadQueue__start_download", new=AsyncMock()):
+        result = await dq.retry(url)
+
+    assert result["status"] == "ok"
+    assert dq.queue.get(url).info.sponsorblock is True
+
+
 @pytest.mark.asyncio
 async def test_add_entry_duplicate_while_pending_is_skipped_not_clobbered(dq_env):
     notifier = AsyncMock()