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/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> =>