feat(scheduler): live admin Scheduler dashboard + reusable poll hook (HU/EN/DE)

New admin Scheduler page (left-nav entry) with a live, self-refreshing view of
job activity, queued work and quota. Polling is factored into a reusable
useLiveQuery hook (pauses when the tab is unfocused) that the notification bell
and future yt-dlp job queue will reuse instead of re-implementing.

frontend/src/components/Header.tsx

@@ -78,7 +78,9 @@ export default function Header({
               ? t("header.account.playlists")
               : page === "settings"
                 ? t("settings.title")
-                : t("header.channelManager")}
+                : page === "scheduler"
+                  ? t("header.scheduler")
+                  : t("header.channelManager")}
         </div>
       )}