fix(errors): surface four mutations that failed silently (C-3.23)

House policy: user actions must report failure. Added onError to:
- ChannelPage block + unsubscribe → notifyYouTubeActionError (a 403 = missing YT
  scope shows the connect message; else the translated fallback);
- SettingsPanel PlexWatchSync toggle + reimport → a plain error notify (these are
  Plex, not YouTube, so notifyYouTubeActionError's "connect YouTube" would mislead);
- DownloadCenter job action (pause/resume/cancel/delete) → a plain error notify.
New keys (HU+EN): channels.notify.blockFailed, settings.plexSync.failed,
downloads.actionFailed.

frontend/src/components/DownloadCenter.tsx

@@ -681,6 +681,7 @@ export default function DownloadCenter() {
             ? api.cancelDownload(id)
             : api.deleteDownload(id),
     onSuccess: invalidate,
+    onError: () => notify({ level: "error", message: t("downloads.actionFailed") }),
   });
 
   // Remove a shared-with-me item from your list (only your grant; the owner's file is untouched).