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/ChannelPage.tsx

@@ -120,7 +120,9 @@ export default function ChannelPage({
       qc.invalidateQueries({ queryKey: ["feed"] });
       qc.invalidateQueries({ queryKey: ["blocked-channels"] });
     },
-    onError: (err) => notifyYouTubeActionError(err, t("channels.notify.blockFailed")),
+    // No onError: block/unblock is a local DB op (not a YouTube write, so no 403 "connect" case),
+    // and its 400/409/422/500 are already surfaced by the global error modal (see api.ts req) —
+    // a caller toast here would just double it.
   });
 
   const subscribe = useMutation({