fix(deferred): close frontend UI/UX tails from the hygiene sweep

- player YB2: keepalive pagehide beacon (api.saveProgressBeacon) so the resume
  position isn't lost on F5/close within 5s of the last checkpoint (mirrors Plex).
- player YB4: bound consecutive unplayable items in auto-advance so an
  auto-advancing/loop=all queue can't spin over a run of dead videos.
- downloads B6 (client): localise structured quota/edit errors centrally in
  api.req() via localizeDetail() + Intl.NumberFormat (fixes HU/DE + decimal sep);
  + 3-locale download error strings.
- channels FB1: focusChannelToken bump so re-clicking the same channel re-seeds
  the search box; FB2: syncSubs invalidates feed/feed-count (set can change now).
- config CB2: reseed the ConfigPanel draft on a key-set dataVersion + explicit
  post-save token, so a mid-edit refetch can't clobber an in-progress edit.
- config AB3 (UI): a blank allow_empty field stores "" instead of resetting.
- admin SB2/SC2: confirm + success toast on demo-whitelist remove and deny-invite
  (+ trilingual strings); SC1: disable only the acted-on row (pendingId), not all;
  SB3: pause the 1s scheduler countdown ticker while the tab is hidden.
- messages CT4: only invalidate conversations/unread when the incoming-message
  count actually changed, not on every 20s poll; MB3 (client): react to the live
  unread ping (onUnread → invalidate); MC3: extract e2ee installKey (setup/unlock);
  CC2: hoist POLL_MS to messaging.ts; MB4: document the reload-scoped socket.

frontend/src/components/Scheduler.tsx

@@ -318,11 +318,33 @@ export default function Scheduler() {
   });
   const data = q.data;
 
-  // Tick once a second so the per-job countdowns move between polls.
+  // Tick once a second so the per-job countdowns move between polls — but only while the tab is
+  // visible; a hidden tab doesn't need to re-render the whole page every second (SB3).
   const [, setTick] = useState(0);
   useEffect(() => {
-    const id = setInterval(() => setTick((n) => n + 1), 1000);
-    return () => clearInterval(id);
+    let id: ReturnType<typeof setInterval> | undefined;
+    const start = () => {
+      if (id == null) id = setInterval(() => setTick((n) => n + 1), 1000);
+    };
+    const stop = () => {
+      if (id != null) {
+        clearInterval(id);
+        id = undefined;
+      }
+    };
+    const onVisibility = () => {
+      if (document.hidden) stop();
+      else {
+        setTick((n) => n + 1); // snap the countdown to the correct value on return
+        start();
+      }
+    };
+    onVisibility(); // honor the current visibility on mount
+    document.addEventListener("visibilitychange", onVisibility);
+    return () => {
+      stop();
+      document.removeEventListener("visibilitychange", onVisibility);
+    };
   }, []);
 
   const pauseResume = useMutation({