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>

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
   }
 }