Fix M3: extract shared recordCompletion() for download-done side-effects

The completion side-effect (add to history unless private + markDownloaded on
the source MediaItem) was duplicated in applyEvent('done') and the preview
fake-ticker, and had already drifted (the preview copy omitted formatId/
formatHasAudio). Extracted one recordCompletion(item) helper that both call.

Also tightened a latent bug: the old applyEvent ran markDownloaded for any
'done' event with a mediaItemId, even when the item was canceled (the reducer
keeps canceled items as 'canceled'). recordCompletion now runs only when
status === 'completed', so a canceled collection item is no longer wrongly
marked downloaded (and will be retried on the next re-sync).

typecheck + 242 tests + eslint green.

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

CODE-AUDIT.md

@@ -223,7 +223,7 @@ token system + shared primitives (UI/SIMP — high value, larger effort) · i18n
   `history.ts`/`errorlog.ts`/`templates.ts` reimplement it inline.
 - [x] **M2 — Remove dead code: `getDefaultFolder` / `download:default-folder`** (channel +
   preload + handler + mock, zero callers).
-- [ ] **M3 — Remove duplicated completion side-effect in `store/downloads.ts`** (history write +
+- [x] **M3 — Remove duplicated completion side-effect in `store/downloads.ts`** (history write +
   `markDownloaded` in both `applyEvent('done')` and the preview ticker). Subsumed by C2.
 - [ ] **M4 — Decide queue persistence explicitly.** Scheduler/queue is renderer-memory only;
   `saved`/scheduled items don't survive a quit. Persist or document as a non-goal in code.

src/renderer/src/store/downloads.ts

@@ -252,6 +252,32 @@ const seed: DownloadItem[] = PREVIEW
     ]
   : []
 
+// Side-effects to run exactly once when a download completes: record it to
+// history (unless private) and mark its source MediaItem downloaded. Shared by
+// the real `applyEvent('done')` path and the preview ticker so the two can't
+// drift (M3).
+function recordCompletion(item: DownloadItem): void {
+  if (!item.incognito) {
+    useHistory.getState().add({
+      id: item.id,
+      title: item.title,
+      channel: item.channel,
+      url: item.url,
+      kind: item.kind,
+      quality: item.quality,
+      formatId: item.formatId,
+      formatHasAudio: item.formatHasAudio,
+      filePath: item.filePath,
+      sizeLabel: item.sizeLabel,
+      thumbnail: item.thumbnail,
+      completedAt: Date.now()
+    })
+  }
+  if (item.mediaItemId) {
+    useSources.getState().markDownloaded(item.mediaItemId, item.filePath)
+  }
+}
+
 // UI-preview only: animate fake progress so the queue feels alive without yt-dlp.
 function startFakeTicker(
   id: string,
@@ -286,25 +312,7 @@ function startFakeTicker(
     if (done) {
       clearInterval(timer)
       const finished = get().items.find((i) => i.id === id)
-      if (finished && !finished.incognito) {
-        useHistory.getState().add({
-          id: finished.id,
-          title: finished.title,
-          channel: finished.channel,
-          url: finished.url,
-          kind: finished.kind,
-          quality: finished.quality,
-          filePath: finished.filePath,
-          sizeLabel: finished.sizeLabel,
-          thumbnail: finished.thumbnail,
-          completedAt: Date.now()
-        })
-      }
-      // Mirror the real applyEvent path so collection completions mark their
-      // source item downloaded in the preview too.
-      if (finished?.mediaItemId) {
-        useSources.getState().markDownloaded(finished.mediaItemId, finished.filePath)
-      }
+      if (finished) recordCompletion(finished)
       get().pump() // a slot just freed -- promote the next queued item
     }
   }, 650)
@@ -609,29 +617,10 @@ export const useDownloads = create<DownloadState>((set, get) => {
           }
         })
       }))
-      // Record completed downloads to history (unless this was a private download).
+      // Record completion (history + source markDownloaded) via the shared helper.
       if (ev.type === 'done') {
         const item = get().items.find((i) => i.id === ev.id)
-        if (item && item.status === 'completed' && !item.incognito) {
-          useHistory.getState().add({
-            id: item.id,
-            title: item.title,
-            channel: item.channel,
-            url: item.url,
-            kind: item.kind,
-            quality: item.quality,
-            formatId: item.formatId,
-            formatHasAudio: item.formatHasAudio,
-            filePath: item.filePath,
-            sizeLabel: item.sizeLabel,
-            thumbnail: item.thumbnail,
-            completedAt: Date.now()
-          })
-        }
-        // Persist media-manager completion so an incremental re-sync skips it.
-        if (item?.mediaItemId) {
-          useSources.getState().markDownloaded(item.mediaItemId, item.filePath)
-        }
+        if (item && item.status === 'completed') recordCompletion(item)
       }
       // A finished item frees a slot -- promote whatever is queued next.
       if (ev.type === 'done' || ev.type === 'error') pump()