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/SettingsPanel.tsx

@@ -533,12 +533,14 @@ function PlexWatchSync() {
     }
   };
 
+  const onFail = () => notify({ level: "error", message: t("settings.plexSync.failed") });
   const toggle = useMutation({
     mutationFn: (next: boolean) => api.plexWatchSetLink(next),
     onSuccess: (res) => {
       qc.setQueryData(["plex-watch-link"], res);
       announce(res.import);
     },
+    onError: onFail,
   });
   const reimport = useMutation({
     mutationFn: () => api.plexWatchImport(),
@@ -546,6 +548,7 @@ function PlexWatchSync() {
       qc.setQueryData(["plex-watch-link"], res);
       announce(res.import);
     },
+    onError: onFail,
   });
 
   const busy = toggle.isPending || reimport.isPending;