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/preload/index.ts

@@ -69,6 +69,9 @@ const api = {
   cancelDownload: (id: string): Promise<void> =>
     ipcRenderer.invoke(IpcChannels.downloadCancel, id),
 
+  pauseDownload: (id: string): Promise<void> =>
+    ipcRenderer.invoke(IpcChannels.downloadPause, id),
+
   getDefaultFolder: (): Promise<string> => ipcRenderer.invoke(IpcChannels.defaultFolder),
 
   chooseFolder: (): Promise<string | null> => ipcRenderer.invoke(IpcChannels.chooseFolder),