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({

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).

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;