Fix L54/L61/L63: thumbnail error fallback, ipcMain.handle, audioQuality label

L54: DownloadBar preview image now tracks failed src in state (same pattern as
     MediaThumb); on load error shows the kind icon fallback instead of broken img.
L61: Convert taskbarProgress from ipcMain.on/ipcRenderer.send to ipcMain.handle/
     ipcRenderer.invoke so all IPC channels use the same request-response idiom.
     App.tsx calls void to explicitly discard the resolved promise.
L63: audioQuality() now has an explicit 'Best' case alongside the bitrate strings;
     unknown labels still fall back to '0' but the default branch is documented.

typecheck + 242 tests + eslint + prettier green.

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

CODE-AUDIT.md

@@ -483,7 +483,7 @@ token system + shared primitives (UI/SIMP — high value, larger effort) · i18n
 - [ ] **L52 — Installer handoff is a fixed `setTimeout(app.quit, 1500)`** (updater.ts) — a race on slow machines.
 - [x] **L53 — `dialog.show*Dialog(win!, …)` non-null assertions** on a possibly-null window
   (chooseFolder, backup export/import) — can throw if the sender window is gone.
-- [ ] **L54 — DownloadBar preview `<img>` has no `onError` fallback** (inconsistent with `MediaThumb`'s retry).
+- [x] **L54 — DownloadBar preview `<img>` has no `onError` fallback** (inconsistent with `MediaThumb`'s retry).
 - [x] **L55 — QueueItem meta line can show size twice** — a probed-format `quality` label already
   contains the size, then `sizeLabel` is appended again.
 - [ ] **L56 — SponsorBlock ON with zero categories silently no-ops** (`buildArgs` guards on
@@ -494,10 +494,10 @@ token system + shared primitives (UI/SIMP — high value, larger effort) · i18n
 - [x] **L59 — `THEME_BACKGROUND` (index.ts) duplicates `pageBackground` (theme.ts)** — two
   hand-synced color constants (a "keep in sync" comment guards them).
 - [x] **L60 — `'external-url'` channel name lacks the `category:` prefix** every other IPC channel uses.
-- [ ] **L61 — `taskbarProgress` is the lone `ipcMain.on`** (fire-and-forget) amid all-`invoke` channels.
+- [x] **L61 — `taskbarProgress` is the lone `ipcMain.on`** (fire-and-forget) amid all-`invoke` channels.
 - [ ] **L62 — "Best available" synthetic format hardcodes `ext:'mp4'`/`hasAudio:true`** (probe.ts) —
   mislabels when the chosen container is mkv/webm.
-- [ ] **L63 — `audioQuality()` unknown label → silent `'0'` (best)** — the audio analog of H5.
+- [x] **L63 — `audioQuality()` unknown label → silent `'0'` (best)** — the audio analog of H5.
 - [ ] **L64 — `ARIA2C_ARGS` connection params hardcoded** (`-x16 -s16 -k1M`), no user control.
 - [ ] **L65 — Pause uses `taskkill /F`** like cancel — a forced kill risks an unflushed `.part` tail vs a graceful stop.
 - [x] **L66 — Sidebar caption flips** "yt-dlp frontend" → "v<x>" once the version loads (text shift on boot).

src/main/buildArgs.ts

@@ -55,8 +55,11 @@ function audioQuality(quality: string): string {
       return '192K'
     case '128 kbps':
       return '128K'
+    case 'Best':
+      return '0'
     default:
-      return '0' // Best
+      // Unrecognised label — fall back to best (0) so the download still works.
+      return '0'
   }
 }
 

src/main/index.ts

@@ -427,8 +427,8 @@ function registerIpcHandlers(): void {
   ipcMain.handle(IpcChannels.scheduledSyncGet, () => getScheduledSync())
   ipcMain.handle(IpcChannels.scheduledSyncSet, (_e, enabled: boolean) => setScheduledSync(enabled))
 
-  // Reflect overall queue progress on the Windows taskbar (fire-and-forget).
-  ipcMain.on(IpcChannels.taskbarProgress, (_e, p: TaskbarProgress) => {
+  // Reflect overall queue progress on the Windows taskbar.
+  ipcMain.handle(IpcChannels.taskbarProgress, (_e, p: TaskbarProgress) => {
     if (!mainWindow || mainWindow.isDestroyed()) return
     if (p.mode === 'none') {
       mainWindow.setProgressBar(-1)

src/preload/index.ts

@@ -223,9 +223,9 @@ const api = {
     return () => ipcRenderer.removeListener(IpcChannels.terminalOutput, listener)
   },
 
-  /** Reflect overall queue progress on the Windows taskbar (fire-and-forget). */
-  setTaskbarProgress: (p: TaskbarProgress): void =>
-    ipcRenderer.send(IpcChannels.taskbarProgress, p),
+  /** Reflect overall queue progress on the Windows taskbar. */
+  setTaskbarProgress: (p: TaskbarProgress): Promise<void> =>
+    ipcRenderer.invoke(IpcChannels.taskbarProgress, p),
 
   /** Open a YouTube WebView to automatically extract a PO token (Phase P). */
   mintPoToken: (): Promise<string | null> =>

src/renderer/src/App.tsx

@@ -69,7 +69,11 @@ function App(): React.JSX.Element {
       lastFraction = s.progress
       lastMode = mode
       lastBadge = s.downloading
-      window.api?.setTaskbarProgress?.({ fraction: s.progress, mode, badgeCount: s.downloading })
+      void window.api?.setTaskbarProgress?.({
+        fraction: s.progress,
+        mode,
+        badgeCount: s.downloading
+      })
     }
     push(useDownloads.getState().items)
     return useDownloads.subscribe((st) => push(st.items))

src/renderer/src/components/DownloadBar.tsx

@@ -327,6 +327,7 @@ export function DownloadBar(): React.JSX.Element {
 
   // Probe state for the single-video format picker.
   const [info, setInfo] = useState<MediaInfo | null>(null)
+  const [thumbFailed, setThumbFailed] = useState<string | null>(null)
   const [probing, setProbing] = useState(false)
   const [probeError, setProbeError] = useState<string | null>(null)
   const [formatId, setFormatId] = useState<string>('')
@@ -627,8 +628,13 @@ export function DownloadBar(): React.JSX.Element {
 
       {info && (
         <div className={styles.preview}>
-          {info.thumbnail ? (
-            <img className={styles.previewThumb} src={info.thumbnail} alt="" />
+          {info.thumbnail && thumbFailed !== info.thumbnail ? (
+            <img
+              className={styles.previewThumb}
+              src={info.thumbnail}
+              alt=""
+              onError={() => setThumbFailed(info.thumbnail ?? null)}
+            />
           ) : (
             <div className={styles.previewThumb}>
               {kind === 'audio' ? (

src/renderer/src/main.tsx

@@ -269,7 +269,7 @@ if (import.meta.env.DEV && !window.api) {
     runTerminal: async () => ({ ok: true }),
     cancelTerminal: async () => {},
     onTerminalOutput: () => () => {},
-    setTaskbarProgress: () => {},
+    setTaskbarProgress: async () => {},
     mintPoToken: async () => null
   }
 }