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/shared/ipc.ts

@@ -19,6 +19,7 @@ export const IpcChannels = {
   probe: 'media:probe',
   downloadStart: 'download:start',
   downloadCancel: 'download:cancel',
+  downloadPause: 'download:pause',
   defaultFolder: 'download:default-folder',
   chooseFolder: 'download:choose-folder',
   openPath: 'shell:open-path',
@@ -158,6 +159,8 @@ export interface DownloadOptions {
   sponsorBlockCategories: SponsorBlockCategory[]
   /** embed chapter markers */
   embedChapters: boolean
+  /** split the download into one file per chapter (--split-chapters) */
+  splitChapters: boolean
   /** embed metadata (title/artist/etc.) */
   embedMetadata: boolean
   /** embed the thumbnail (cover art for audio, poster for mp4/mkv video) */
@@ -183,6 +186,7 @@ export const DEFAULT_DOWNLOAD_OPTIONS: DownloadOptions = {
   sponsorBlockMode: 'remove',
   sponsorBlockCategories: ['sponsor'],
   embedChapters: false,
+  splitChapters: false,
   embedMetadata: true,
   embedThumbnail: true,
   cropThumbnail: false,
@@ -389,6 +393,13 @@ export interface StartDownloadOptions {
    * download even when the settings default is enabled.
    */
   extraArgs?: string
+  /**
+   * Optional trim: keep only these time ranges. Raw user text — ranges like
+   * '1:30-2:00', comma- or newline-separated — normalised to yt-dlp
+   * `--download-sections` specs in buildArgs (parseTrimSections). Empty or
+   * undefined downloads the whole media.
+   */
+  trim?: string
   /**
    * Metadata the renderer already fetched when probing the URL (title/channel/
    * duration). When present, main uses it directly instead of spawning a second