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/ChannelPage.tsx

@@ -120,6 +120,7 @@ export default function ChannelPage({
       qc.invalidateQueries({ queryKey: ["feed"] });
       qc.invalidateQueries({ queryKey: ["blocked-channels"] });
     },
+    onError: (err) => notifyYouTubeActionError(err, t("channels.notify.blockFailed")),
   });
 
   const subscribe = useMutation({
@@ -144,6 +145,7 @@ export default function ChannelPage({
       qc.invalidateQueries({ queryKey: ["feed"] });
       qc.invalidateQueries({ queryKey: ["channels"] });
     },
+    onError: (err) => notifyYouTubeActionError(err, t("channels.notify.unsubscribeFailed")),
   });
   const onUnsubscribe = async () => {
     const ok = await confirm({