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/download.ts

@@ -38,6 +38,7 @@ import {
 interface ActiveDownload {
   child: ChildProcess
   canceled: boolean
+  paused: boolean
 }
 
 const active = new Map<string, ActiveDownload>()
@@ -295,7 +296,7 @@ export function startDownload(
     return { ok: false, error: (e as Error).message }
   }
 
-  const rec: ActiveDownload = { child, canceled: false }
+  const rec: ActiveDownload = { child, canceled: false, paused: false }
   active.set(opts.id, rec)
 
   // Title/channel/duration are kept locally so the completion notification and
@@ -344,7 +345,8 @@ export function startDownload(
     if (settled) return
     settled = true
     active.delete(opts.id)
-    if (!rec.canceled) {
+    // A paused download was killed on purpose — stay silent, like a cancel.
+    if (!rec.canceled && !rec.paused) {
       send(wc, { type: 'error', id: opts.id, error: err.message })
       logFailure(opts, resolvedTitle, err.message)
       notify(wc, resolvedTitle ?? 'Download failed', err.message)
@@ -355,7 +357,9 @@ export function startDownload(
     if (settled) return
     settled = true
     active.delete(opts.id)
-    if (rec.canceled) return // renderer already showed 'canceled' optimistically
+    // Canceled: renderer already showed 'canceled'. Paused: renderer showed
+    // 'paused' and keeps the .part for a later resume. Either way, no event.
+    if (rec.canceled || rec.paused) return
     if (code === 0) {
       send(wc, { type: 'done', id: opts.id, filePath })
       notify(wc, resolvedTitle ?? 'Download complete', 'Finished downloading.')
@@ -370,15 +374,12 @@ export function startDownload(
   return { ok: true }
 }
 
-export function cancelDownload(id: string): void {
-  const rec = active.get(id)
-  if (!rec) return
-  rec.canceled = true
+// Kill the whole process tree (/T) so the spawned ffmpeg child dies too. Resolve
+// taskkill from System32 by absolute path, never the bare name, so a planted
+// taskkill.exe on PATH / in the CWD can't run in its place. (audit F3)
+function killTree(rec: ActiveDownload): void {
   const pid = rec.child.pid
   if (pid != null) {
-    // Kill the whole tree (/T) so the spawned ffmpeg child dies too. Resolve
-    // taskkill from System32 by absolute path, never the bare name, so a planted
-    // taskkill.exe on PATH / in the CWD can't run in its place. (audit F3)
     execFile(
       getSystem32Path('taskkill.exe'),
       ['/pid', String(pid), '/T', '/F'],
@@ -389,3 +390,24 @@ export function cancelDownload(id: string): void {
     rec.child.kill()
   }
 }
+
+export function cancelDownload(id: string): void {
+  const rec = active.get(id)
+  if (!rec) return
+  rec.canceled = true
+  killTree(rec)
+}
+
+/**
+ * Pause a running download: kill the process tree but flag it `paused` (not
+ * `canceled`) so the close handler stays silent — no error event, no error log,
+ * no notification. yt-dlp leaves the partial `.part` file in place, so resuming
+ * is just a fresh startDownload with the same options: yt-dlp's default
+ * `--continue` picks the partial file back up.
+ */
+export function pauseDownload(id: string): void {
+  const rec = active.get(id)
+  if (!rec) return
+  rec.paused = true
+  killTree(rec)
+}