feat: queue UX (Phase M) + media editing (split-chapters, trim)

Phase L (media editing):
- Split by chapters (--split-chapters) as a DownloadOptions toggle
- Trim/cut by time range: parseTrimSections -> --download-sections "*A-B"
  + --force-keyframes-at-cuts (deduped vs SponsorBlock), per-download Trim panel

Phase M (queue & daily-use UX), all 7:
- Pause/resume: main-side killTree + paused flag (silent close), download:pause
  IPC, store pause/resume + paused status, QueueItem controls
- Reorder via "Download next" (prioritize)
- Aggregate progress strip (pure summarizeQueue) + combined speed/ETA
- Drag-and-drop links / .url files onto the download card
- Retry all failed
- Duplicate detection (sameVideo) with "Download anyway" guard
- Per-download scheduling (datetime-local) + save-for-later (saved status,
  15s promotion ticker); session-only (queue not persisted)

New pure modules unit-tested (queueStats); buildArgs trim/split tested.
typecheck + test green (178 passed). Roadmap updated: Phase M COMPLETE.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/main/settings.ts

@@ -111,6 +111,7 @@ function sanitizeOptions(input: unknown): DownloadOptions {
     sponsorBlockMode: o.sponsorBlockMode === 'mark' ? 'mark' : 'remove',
     sponsorBlockCategories: cats,
     embedChapters: bool(o.embedChapters, d.embedChapters),
+    splitChapters: bool(o.splitChapters, d.splitChapters),
     embedMetadata: bool(o.embedMetadata, d.embedMetadata),
     embedThumbnail: bool(o.embedThumbnail, d.embedThumbnail),
     cropThumbnail: bool(o.cropThumbnail, d.cropThumbnail),