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/index.ts

@@ -14,7 +14,7 @@ import { getYtdlpVersion, updateYtdlp, runStartupYtdlpAutoUpdate } from './ytdlp
 import { getFfmpegVersions } from './ffmpeg'
 import { checkForAppUpdate, downloadAppUpdate, runAppUpdate } from './updater'
 import { probeMedia } from './probe'
-import { startDownload, cancelDownload, previewCommand } from './download'
+import { startDownload, cancelDownload, pauseDownload, previewCommand } from './download'
 import { getSettings, setSettings, ensureMediaDirs } from './settings'
 import { listHistory, addHistory, removeHistory, removeManyHistory, clearHistory } from './history'
 import { listTemplates, saveTemplate, removeTemplate } from './templates'
@@ -211,6 +211,7 @@ function registerIpcHandlers(): void {
     return result
   })
   ipcMain.handle(IpcChannels.downloadCancel, (_e, id: string) => cancelDownload(id))
+  ipcMain.handle(IpcChannels.downloadPause, (_e, id: string) => pauseDownload(id))
 
   ipcMain.handle(IpcChannels.defaultFolder, () => app.getPath('downloads'))