fix(errors,a11y): review round 1 — drop redundant onErrors, stabilize the live region

Review found my C-3.23 additions double-notified: the global error modal (api.ts req)
already surfaces 400/409/422/500 with the backend's specific message, so a caller
toast on those codes is a second, weaker surface. Only 403/404 are caller-handled and
silent — and only a YouTube WRITE action has a meaningful 403 (missing scope).

So the correct set is just the ChannelPage unsubscribe handler (a YouTube write, 403 →
"connect", matching subscribe). Reverted the redundant onErrors on block (a local DB
op, not YouTube), SettingsPanel Plex watch toggle/reimport, and the DownloadCenter job
action — the global modal already speaks for them. Removed the three now-unused keys.

Toaster: moved the polite live region onto the stable, always-mounted container (a
polite region must pre-exist to announce later insertions); error/fatal toasts keep
role="alert", which announces on insertion regardless.

frontend/src/components/SettingsPanel.tsx

@@ -533,14 +533,15 @@ function PlexWatchSync() {
     }
   };
 
-  const onFail = () => notify({ level: "error", message: t("settings.plexSync.failed") });
+  // No onError on these: they're Plex, not YouTube (notifyYouTubeActionError would wrongly say
+  // "connect YouTube"), and their 409/500 are already surfaced by the global error modal — a caller
+  // toast would double it. Actual import failures come back as 200 {error}, handled by announce.
   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(),
@@ -548,7 +549,6 @@ function PlexWatchSync() {
       qc.setQueryData(["plex-watch-link"], res);
       announce(res.import);
     },
-    onError: onFail,
   });
 
   const busy = toggle.isPending || reimport.isPending;