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/Messages.tsx

@@ -5,7 +5,7 @@ import { ArrowLeft, ShieldCheck, SquarePen, ExternalLink } from "lucide-react";
 import { api, type Conversation, type MessageUser } from "../lib/api";
 import { useLiveQuery } from "../lib/useLiveQuery";
 import { relativeTime } from "../lib/format";
-import { partnerPub, SYSTEM_ID, useKeyState } from "../lib/messaging";
+import { partnerPub, POLL_MS, SYSTEM_ID, useKeyState } from "../lib/messaging";
 import { useHistorySubview } from "../lib/history";
 import { openChat } from "../lib/chatDock";
 import * as e2ee from "../lib/e2ee";
@@ -13,8 +13,6 @@ import Avatar from "./Avatar";
 import KeyGate from "./KeyGate";
 import ChatThread from "./ChatThread";
 
-const POLL_MS = 20000;
-
 type View = "list" | "new" | { partnerId: number; name?: string; avatar?: string | null; system?: boolean };
 
 export default function Messages({ meId }: { meId: number }) {