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>

ui/src/app/services/downloads.service.ts

@@ -17,6 +17,7 @@ export interface AddDownloadPayload {
   playlistItemLimit: number;
   autoStart: boolean;
   splitByChapters: boolean;
+  sponsorblock: boolean;
   chapterTemplate: string;
   subtitleLanguage: string;
   subtitleMode: string;
@@ -148,6 +149,7 @@ export class DownloadsService {
       playlist_item_limit: payload.playlistItemLimit,
       auto_start: payload.autoStart,
       split_by_chapters: payload.splitByChapters,
+      sponsorblock: payload.sponsorblock,
       chapter_template: payload.chapterTemplate,
       subtitle_language: payload.subtitleLanguage,
       subtitle_mode: payload.subtitleMode,