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/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)